selftests/user_events: Fix failures when user_events is not installed

When user_events is not installed the self tests currently fail. Now
that these self tests run by default we need to ensure they don't fail
when user_events was not enabled for the kernel being tested.

Add common methods to detect if tracefs and user_events is enabled. If
either is not enabled skip the test. If tracefs is enabled, but is not
mounted, mount tracefs and fail if there were any errors. Fail if not
run as root.

Fixes: 68b4d2d5 ("selftests/user_events: Reenable build")
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Link: https://lore.kernel.org/all/CA+G9fYuugZ0OMeS6HvpSS4nuf_A3s455ecipGBvER0LJHojKZg@mail.gmail.com/



Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>