Add run-tests.php --context [n] option.

[TysonAndre]

Mentioned in #5965 (comment)

This PR proposes 3 lines of context so the impact can be seen in tests.
Other diff programs show around 3 lines of context.
(This helps indicate exactly which position a test should be updated
to add a new expected line at)

Are there reasons to continue not printing lines of context for test failures?
The lack of context seems inconvenient when investigating CI failures or when the line order wasn't obvious.

[TysonAndre]

Example output can be seen at https://travis-ci.org/github/php/php-src/jobs/717434379 . Lines starting with -- were used as a placeholder in this PR to indicate where common lines of context end.

The line numbers will take up more columns for additions or removals in all places if either the expected or actual output take up more than 999 lines.

This PR was created before the PR to colorize diff output was merged.

@nikic @Girgias

========DIFF========
     test_time_spring_type3_prev_type2_prev: DIFF: 2010-03-13 18:38:28 EST - 2010-02-11 02:18:48 EST = **P+0Y1M2DT16H19M40S**
     test_time_spring_type3_prev_type2_st: DIFF: 2010-03-14 00:10:20 EST - 2010-03-13 18:38:28 EST = **P+0Y0M0DT5H31M52S**
     test_time_spring_type3_prev_type2_dt: DIFF: 2010-03-14 03:16:55 EDT - 2010-03-13 18:38:28 EST = **P+0Y0M0DT7H38M27S**
004+ test_time_spring_type3_prev_type2_post: DIFF: 2010-03-15 19:59:59 EDT - 2010-03-13 18:38:28 EST = **P+0Y0M2DT0H21M31S**
004- test_time_spring_type3_prev_type2_post: DIFF: 2010-03-15 19:59:59 EDT - 2010-03-13 18:38:28 EST = **P+0Y0M2DT1H21M31S**
     test_time_spring_type3_st_type2_prev: DIFF: 2010-03-13 18:38:28 EST - 2010-03-14 00:10:20 EST = **P-0Y0M0DT5H31M52S**
     test_time_spring_type3_st_type2_st: DIFF: 2010-03-14 00:15:35 EST - 2010-03-14 00:10:20 EST = **P+0Y0M0DT0H5M15S**
     test_time_spring_type3_st_type2_dt: DIFF: 2010-03-14 03:16:55 EDT - 2010-03-14 00:10:20 EST = **P+0Y0M0DT2H6M35S**
--
     test_time_spring_type3_dt_type2_st: DIFF: 2010-03-14 00:10:20 EST - 2010-03-14 03:16:55 EDT = **P-0Y0M0DT2H6M35S**
     test_time_spring_type3_dt_type2_dt: DIFF: 2010-03-14 05:19:56 EDT - 2010-03-14 03:16:55 EDT = **P+0Y0M0DT2H3M1S**
     test_time_spring_type3_dt_type2_post: DIFF: 2010-03-15 19:59:59 EDT - 2010-03-14 03:16:55 EDT = **P+0Y0M1DT16H43M4S**
013+ test_time_spring_type3_post_type2_prev: DIFF: 2010-03-13 18:38:28 EST - 2010-03-15 19:59:59 EDT = **P-0Y0M2DT0H21M31S**
013- test_time_spring_type3_post_type2_prev: DIFF: 2010-03-13 18:38:28 EST - 2010-03-15 19:59:59 EDT = **P-0Y0M2DT1H21M31S**
     test_time_spring_type3_post_type2_st: DIFF: 2010-03-14 00:10:20 EST - 2010-03-15 19:59:59 EDT = **P-0Y0M1DT18H49M39S**
     test_time_spring_type3_post_type2_dt: DIFF: 2010-03-14 03:16:55 EDT - 2010-03-15 19:59:59 EDT = **P-0Y0M1DT16H43M4S**
     test_time_spring_type3_post_type2_post: DIFF: 2010-03-15 19:59:59 EDT - 2010-03-15 18:57:55 EDT = **P+0Y0M0DT1H2M4S**
017+ test_time_spring_type3_stsec_type2_dtsec: DIFF: 2010-03-14 03:00:00 EDT - 2010-03-14 01:59:59 EST = **P+0Y0M0DT0H0M1S**
018+ test_time_spring_type3_dtsec_type2_stsec: DIFF: 2010-03-14 01:59:59 EST - 2010-03-14 03:00:00 EDT = **P-0Y0M0DT0H0M1S**
017- test_time_spring_type3_stsec_type2_dtsec: DIFF: 2010-03-15 03:00:00 EDT - 2010-03-13 01:59:59 EST = **P+0Y0M0DT0H0M1S**
018- test_time_spring_type3_dtsec_type2_stsec: DIFF: 2010-03-15 01:59:59 EST - 2010-03-15 03:00:00 EDT = **P-0Y0M0DT0H0M1S**
========DONE========
XFAIL DateTime::diff() -- spring type3 type2 [ext/date/tests/DateTime_diff-spring-type3-type2.phpt]   XFAIL REASON: Various bugs exist

The expectation of DateTime_diff-spring-type3-type2.phpt

--EXPECT--
test_time_spring_type3_prev_type2_prev: DIFF: 2010-03-13 18:38:28 EST - 2010-02-11 02:18:48 EST = **P+0Y1M2DT16H19M40S**
test_time_spring_type3_prev_type2_st: DIFF: 2010-03-14 00:10:20 EST - 2010-03-13 18:38:28 EST = **P+0Y0M0DT5H31M52S**
test_time_spring_type3_prev_type2_dt: DIFF: 2010-03-14 03:16:55 EDT - 2010-03-13 18:38:28 EST = **P+0Y0M0DT7H38M27S**
test_time_spring_type3_prev_type2_post: DIFF: 2010-03-15 19:59:59 EDT - 2010-03-13 18:38:28 EST = **P+0Y0M2DT1H21M31S**
test_time_spring_type3_st_type2_prev: DIFF: 2010-03-13 18:38:28 EST - 2010-03-14 00:10:20 EST = **P-0Y0M0DT5H31M52S**
test_time_spring_type3_st_type2_st: DIFF: 2010-03-14 00:15:35 EST - 2010-03-14 00:10:20 EST = **P+0Y0M0DT0H5M15S**
test_time_spring_type3_st_type2_dt: DIFF: 2010-03-14 03:16:55 EDT - 2010-03-14 00:10:20 EST = **P+0Y0M0DT2H6M35S**
test_time_spring_type3_st_type2_post: DIFF: 2010-03-15 19:59:59 EDT - 2010-03-14 00:10:20 EST = **P+0Y0M1DT18H49M39S**
test_time_spring_type3_dt_type2_prev: DIFF: 2010-03-13 18:38:28 EST - 2010-03-14 03:16:55 EDT = **P-0Y0M0DT7H38M27S**
test_time_spring_type3_dt_type2_st: DIFF: 2010-03-14 00:10:20 EST - 2010-03-14 03:16:55 EDT = **P-0Y0M0DT2H6M35S**
test_time_spring_type3_dt_type2_dt: DIFF: 2010-03-14 05:19:56 EDT - 2010-03-14 03:16:55 EDT = **P+0Y0M0DT2H3M1S**
test_time_spring_type3_dt_type2_post: DIFF: 2010-03-15 19:59:59 EDT - 2010-03-14 03:16:55 EDT = **P+0Y0M1DT16H43M4S**
test_time_spring_type3_post_type2_prev: DIFF: 2010-03-13 18:38:28 EST - 2010-03-15 19:59:59 EDT = **P-0Y0M2DT1H21M31S**
test_time_spring_type3_post_type2_st: DIFF: 2010-03-14 00:10:20 EST - 2010-03-15 19:59:59 EDT = **P-0Y0M1DT18H49M39S**
test_time_spring_type3_post_type2_dt: DIFF: 2010-03-14 03:16:55 EDT - 2010-03-15 19:59:59 EDT = **P-0Y0M1DT16H43M4S**
test_time_spring_type3_post_type2_post: DIFF: 2010-03-15 19:59:59 EDT - 2010-03-15 18:57:55 EDT = **P+0Y0M0DT1H2M4S**
test_time_spring_type3_stsec_type2_dtsec: DIFF: 2010-03-15 03:00:00 EDT - 2010-03-13 01:59:59 EST = **P+0Y0M0DT0H0M1S**
test_time_spring_type3_dtsec_type2_stsec: DIFF: 2010-03-15 01:59:59 EST - 2010-03-15 03:00:00 EDT = **P-0Y0M0DT0H0M1S**

[Girgias]

LGTM and can indeed help while debugging tests failures on CI.

Just FYI Nikita is currently on vacation so if you want his input, you will need to wait.