selftests/pidfd_test: Remove the erroneous ','

Remove the erroneous ',', otherwise it might result in wrong output
and report:
 ...
 Bail out! (errno %d)
  test: Unexpected epoll_wait result (c=4208480, events=2)
 ...

Fixes: 740378dc ("pidfd: add polling selftests")
Signed-off-by: Zhao Gongyi <zhaogongyi@huawei.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>