selftests: ftrace: Have some basic tests run in a tracing instance too

Some of the basic ftrace selftests should also be run in an instance. These
test a quick case of running all tracers in the available_tracers file
within the instance. The other is testing the clock used for the instance.

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