selftests/ftrace: Stop tracing while reading the trace file by default

Stop tracing while reading the trace file by default, to prevent
the test results while checking it and to avoid taking a long time
to check the result.
If there is any testcase which wants to test the tracing while reading
the trace file, please override this setting inside the test case.

This also recovers the pause-on-trace when clean it up.

Link: https://lkml.kernel.org/r/163529053143.690749.15365238954175942026.stgit@devnote2



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