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>

tools/testing/selftests/ftrace/test.d/event/event-enable.tc

 #!/bin/sh
 # description: event tracing - enable/disable with event level files
+# flags: instance
 
 do_reset() {
     echo > set_event

tools/testing/selftests/ftrace/test.d/event/event-pid.tc

 #!/bin/sh
 # description: event tracing - restricts events based on pid
+# flags: instance
 
 do_reset() {
     echo > set_event

tools/testing/selftests/ftrace/test.d/event/subsystem-enable.tc

 #!/bin/sh
 # description: event tracing - enable/disable with subsystem level files
+# flags: instance
 
 do_reset() {
     echo > set_event