testCompilation doesn't work with separate compilation anymore due to -Ytest-pickler

[biboudis]

$ cp -r tests/pos/i4023 tests/pos/i4024
sbt> testCompilation i4023 
sbt> testCompilation i4024

First testCompilation is OK, second raises compilation/classpath error (i4023 is used for demonstration purposes). Reproduced on master on mine and @liufengyun's PC.

[biboudis]

A note at #5059 revealed that this is an issue with -Ytest-pickler and the directories that use separate compilation tests.

@allanrenucci can we afford running once with -Ytest-pickler and once without for each file that participates in a SeparateCompilationSource (because test pickler stops compilation)?

[liufengyun]

Fixed in #5571 -- in pickling tests, separate compilation only test the first group.