suffix in extra test outputs

[Denis Davydov]

Dear all,

I would like to suggest that we add some sensible info into suffix when adding

extra test output. For example

fragile_test.output

fragile_test.output.gcc6

fragile_test.ouptut.gcc4

fragile_test.output.clang8-sierra

When we have fragile tests with 3-4 different outputs, it is easier to "read" them if the filenames have some hints.

Ideally if the test fails on some system/compiler, one hopefully would have a clue whether or not it's because the test is fragile

or there is some problem indeed.

Note that commit messages may not contain any info on rationale why the extra outputs were added.

In the long run, i think it's better to document it in both places -- commit message and brief info in filename.

Regards,

Denis