[openmp][test] Don't use random Flang by default

Only enable Fortran tests when either the test compiler is set
explicitly, or in a runtimes bootstrapping build. A system-installed
Flang either may not exist, or too old to compiler our tests.

Fixes buildbot failure after landing #150722
https://lab.llvm.org/buildbot/#/builders/10/builds/13905

GitOrigin-RevId: 22a64170df0365c53d004a7c763d83776aa065a9