test: try to detect SIGILL in stress-ng and skip TEST-55-OOMD gracefully (#42290)

The attempt to fix SIGILL in TEST-55-OOMD actually made the test more
flaky, as the chosen stress-ng method generates less pressure, so it
often fails.

Try a different approach: try to detect that SIGILL caused the unit
calling stress-ng to fail, and skip gracefully in that case.