selftests: ftrace: Initialize ftrace before each test

Reset ftrace to initial state before running each test.
This fixes some test cases to enable tracing before starting
trace test. This can avoid false-positive failure when
previous testcase fails while disabling tracing.

Link: http://lkml.kernel.org/r/147928398192.22982.7767460638302113002.stgit@devbox



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