selftests/perf_events: Test modification of perf_event_attr::sig_data

Test that PERF_EVENT_IOC_MODIFY_ATTRIBUTES correctly modifies
perf_event_attr::sig_data as well.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Link: https://lore.kernel.org/r/20220131103407.1971678-2-elver@google.com