rtla/tests: Add $SUBTESTS and $STOPONFAIL

Give the user more control over the test execution when the test scripts
are run directly (not through prove), in order to do things like run a
particular test repeatedly to diagnose an intermittent failure.

If $SUBTESTS is provided, then only tests whose number is on the list will
run.  For example:

  # SUBTESTS="14 133 4 $(seq 98 103) 9" bash ./tests/timerlat.t
  1..150
  ok 4 - top verify -s/--stack
  ok 9 - hist set the automatic trace mode
  ok 14 - top disable auto-analysis
  ok 98 - hist verify -H/--house-keeping
  ok 99 - top verify -k/--kernel-threads
  ok 100 - hist verify -k/--kernel-threads
  ok 101 - top verify -u/--user-threads
  ok 102 - hist verify -u/--user-threads
  ok 103 - apply default timerlat_period_us
  ok 133 - top trace output through --on-threshold trace

If $STOPONFAIL is set, then the script will exit with a non-zero exit code
on the first failure.  If the output directory is non-empty, it will be
preserved, and the location shown.

Signed-off-by: Crystal Wood <crwood@redhat.com>
Link: https://lore.kernel.org/r/20260919062756.819701-4-crwood@redhat.com
Signed-off-by: Tomas Glozar <tglozar@redhat.com>