Add an ISO-8601 timestamp to every testsuite and testcase in the XML-report

[fniessink]

To monitor when tests have last run, we read the timestamp attribute from JUnit-XML files. It would be nice if XML generated by XMLRunner included this timestamp attribute.

This pull request adds timestamps to testsuites and testcases in the XML output. The timestamp attributes are ISO8601 as specified by the JUnit DTD included in the XMLRunner sources.

[coveralls]

Coverage increased (+0.005%) to 99.301% when pulling cce91c2 on ICTU:master into 8e53812 on xmlrunner:master.

[codecov-io]

Codecov Report

Merging #157 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #157      +/-   ##
=========================================
+ Coverage   99.29%   99.3%   +<.01%     
=========================================
  Files          17      17              
  Lines        1136    1151      +15     
=========================================
+ Hits         1128    1143      +15     
  Misses          8       8

Impacted Files	Coverage Δ
tests/builder_test.py	100% <100%> (ø)	⬆️
xmlrunner/builder.py	100% <100%> (ø)	⬆️
xmlrunner/result.py	97.61% <100%> (+0.05%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8e53812...05a94fc. Read the comment docs.

[coveralls]

Coverage increased (+0.009%) to 99.305% when pulling 05a94fc on ICTU:master into 8e53812 on xmlrunner:master.