selftests/user_events: Ensure auto cleanup works as expected

User events now auto cleanup upon the last reference put. Update
ftrace_test to ensure this works as expected. Ensure EBUSY delays
while event is being deleted do not cause transient failures by
waiting and re-attempting.

Link: https://lkml.kernel.org/r/20230614163336.5797-5-beaub@linux.microsoft.com



Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>