selftests: ftrace: Make func_event_triggers and func_traceonoff_triggers tests do instances

Both the func_event_triggers and func_traceonoff_triggers tests can be
performed in both the toplevel instance as well as for individual instances.
Have their tests run in both cases.

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