selftests/ftrace: Improve integration with kselftest runner

The ftrace selftests do not currently produce KTAP output, they produce a
custom format much nicer for human consumption. This means that when run in
automated test systems we just get a single result for the suite as a whole
rather than recording results for individual test cases, making it harder
to look at the test data and masking things like inappropriate skips.

Address this by adding support for KTAP output to the ftracetest script and
providing a trivial wrapper which will be invoked by the kselftest runner
to generate output in this format by default, users using ftracetest
directly will continue to get the existing output.

This is not the most elegant solution but it is simple and effective. I
did consider implementing this by post processing the existing output
format but that felt more complex and likely to result in all output being
lost if something goes seriously wrong during the run which would not be
helpful. I did also consider just writing a separate runner script but
there's enough going on with things like the signal handling for that to
seem like it would be duplicating too much.

Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Tested-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>