selftests/ftrace: Add case number prefix to logfile

Add a case number prefix to each logfile. This makes
it easier to find which logfile is corresponding
to which failure.

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>