selftests: ftrace: Allow some event trigger tests to run in an instance

Some of the event triggers can run fine in an instance. Have them tested in
one as well. The ones that still need work are the snapshot, stacktrace and
traceon/off triggers, as they don't currently pass a handle to the
trace_array they are attached to. But that can be for a future project.

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