rtla/tests: Add get_workload_pids() helper

rtla/tests: Add get_workload_pids() helper

RTLA runtime tests that check workload processes (currently the test
case "verify -P/--priority" of timerlat.t and "verify the --priority/-P
param" of osnoise.t) use "pgrep timerlatu/" or "pgrep osnoise/"
respectively to identify the workload.

Make them more robust by adding a get_workload_pids() helper that
finds the main rtla process and returns the PIDs of all siblings other
than the test script itself, plus all child processes of kthreadd that
have the osnoise/timerlat kthread pattern comm.

This filters out any spurious processes not related to the running test
that happen to have "timerlatu/" or "osnoise/" in their command, for
example, a user grepping the same names at the time of the running of
the test.

Reviewed-by: Wander Lairson Costa <wander@redhat.com>
Link: https://lore.kernel.org/r/20260423130558.882022-3-tglozar@redhat.com
Signed-off-by: Tomas Glozar <tglozar@redhat.com>