rtla/tests: Fix pgrep filter in get_workload_pids.sh

rtla/tests: Fix pgrep filter in get_workload_pids.sh

Multiple runtime tests in RTLA rely on the get_workload_pids() shell
helper function to get the PIDs of both kernel and user workloads.

On some systems (e.g. Fedora 43), pgrep matches kernel thread names
including square brackets: "[osnoise/0]"; on other systems (e.g.
RHEL 9.8), brackets are not included: "osnoise/0".

Accept both as valid workload PIDs rather that just the non-bracket form
to make the tests work on all systems.

Fixes: a98dad63cda3 ("rtla/tests: Add runtime test for -k and -u options")
Reported-by: Crystal Wood <crwood@redhat.com>
Link: https://lore.kernel.org/r/20260604140547.3616495-1-tglozar@redhat.com
Signed-off-by: Tomas Glozar <tglozar@redhat.com>