selftests/ftrace: Cleanup ftrace after running test

Cleanup ftrace by initialize_ftrace() after running
all test cases. This means we also don't need cleanup
ftrace on each test case, except for some special
options.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>