selftests/ftrace: Add --stop-fail hidden option for debug

Add --stop-fail option for debugging the ftracetest.

With this option, ftracetest stops right after a testcase
fails instead of finish running all testcases.

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>