perf test: Be more tolerant of metricgroup failures

Previously "set -e" meant any non-zero exit code from perf stat would
cause a test failure. As a non-zero exit happens when there aren't
sufficient permissions, check for this case and make the exit code
2/skip for it.

Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Veronika Molnarova <vmolnaro@redhat.com>
Cc: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Link: https://lore.kernel.org/r/20240502223115.2357499-1-irogers@google.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>