selftests/perf_events: Add kselftest for remove_on_exec

Add kselftest to test that remove_on_exec removes inherited events from
child tasks.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210408103605.1676875-9-elver@google.com