tracing/selftest: Add test to test simple snapshot trigger for trace_marker

Several complex trigger tests were added for trace_marker, but not a simple
one. This could be used to help diagnose a problem with the code by giving a
reference between how complex a trigger is that fails.

Suggested-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>