[compiler-rt][test] Make instrprof-set-dir-mode test tolerant of group ID

Patch from 'troyj':
Hi, I ran into a problem with this test when the source was located in
certain directories. The mkdir(2) man page states that the set-group-ID
bit is inherited from the parent directory, but this test was written in
such a way that it assumes the bit is unset. Whether that assumption is
true depends on where the checkout lives, which leads to some people
being able to reproduce the problem whereas others cannot. I think the
correct fix is to exclude the bit from the check.

Making probinson a reviewer since they reviewed the original test.

Patch landed for troyj, thanks!

Differential Revision: D53832

llvm-svn: 357449