selftests/ftrace: Replace echo -e with printf

echo -e is not POSIX. Depending on what /bin/sh is, we can get
incorrect output like:
$ -e -n [1] Basic trace file check
$ -e 	[PASS]

Fix that by using printf instead.

Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Shuah Khan <shuah@kernel.org>