selftests: add epoll selftests

selftests: add epoll selftests

This adds the promised selftest for epoll.  It will verify the wakeups
of epoll.  Including leaf and nested mode, epoll_wait() and poll() and
multi-threads.

Link: http://lkml.kernel.org/r/20191009121518.4027-1-r@hev.cc
Signed-off-by: hev <r@hev.cc>
Reviewed-by: Roman Penyaev <rpenyaev@suse.de>
Cc: Jason Baron <jbaron@akamai.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>