selftests: ftrace: Allow some tests to be run in a tracing instance

An tracing instance has several of the same capabilities as the top level
instance, but may be implemented slightly different. Instead of just writing
tests that duplicat the same test cases of the top level instance, allow a
test to be written for both the top level as well as for an instance.

If a test case can be run in both the top level as well as in an tracing
instance directory, then it should add a tag "# flags: instance" in the
header of the test file. Then after all tests have run, any test that has an
instance flag set, will run again within a tracing instance.

Link: http://lkml.kernel.org/r/20170421233850.1d0e9e05@gandalf.local.home



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