selftests: ftrace: Have event tests also run in an tracing instance

The ftrace selftests of events: event-enable, event-pid, and
subsystem-enable can all be run inside an instance. Change their tests to do
both a toplevel run an an instance run.

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