selftests: tracing: Enable fprobe events before checking enable_functions

Since the fprobe is not registered before enabling the fprobe events,
enable_functions is also empty before enabling it. Thus the tests
which checking enable_functions must ensure the event is enabled before
testing the enable_functions.

Link: https://lore.kernel.org/all/174343538009.843280.6583146613234713007.stgit@devnote2/



Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>