selftests/ftrace: Strip escape sequences for log file

Strip escape sequences from the stream to the ftracetest
summary log file. Note that all test-case results are
dumped raw in each file.

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>