selftests/ftrace: More initialize features in initialize_ftrace

Clear pid filter, synthetic_events, snapshots,
ftrace filter, and trace log in initialize_ftrace(),
since those are used in test cases.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>