test: skip TEST-55-OOMD entirely if stress-ng is broken on this host

test: skip TEST-55-OOMD entirely if stress-ng is broken on this host

This reverts commit a17efef137 ("test: try to detect SIGILL in stress-ng
and skip TEST-55-OOMD gracefully") and replaces it with a single
check to skip the test cases.

The previous check was not reliable as stress-ng can catch SIGILL
itself and exist with an error:

  stress-ng[1068]: stress-ng: debug: [1068] caught SIGILL, address \
    0x00005632f8330140 (ILL_ILLOPN)
  stress-ng[1068]: stress-ng: debug: [1068] stress-ng: info: \
    0x00005632f8330140:<62>71 fd 48 6f 2d 36 14 1c 00 c5 d1 ef ed 49 29
  ...
  stress-ng[1053]: stress-ng: error: [1053] vm: [1061] terminated \
    with an error, exit status=2 (stressor failed)
  ...
  systemd[1]: TEST-55-OOMD-slowrule.service: Main process exited, \
    code=exited, status=2/INVALIDARGUMENT
  systemd[1]: TEST-55-OOMD-slowrule.service: Failed with result \
    'exit-code'.

Try to detect at the beginning of the test and skip the test case if it
happens.