[flang] Fix flang tests on MacOS (#70811)

Adjust some of the tests run by check-flang to make them pass on MacOS,
either by skipping unsupported tests or by adapting the test for correct
execution on MacOS.

For now ctofortran.f90 test is marked as unsupported, but it can be
adapted to run on MacOS once support for -isysroot flag is added to
flang.

Issues #70805 and #70807 are tracking the failing tests that remain, as
these reveal real problems with flang.