selftests/ftrace: Make XFAIL green color

Since XFAIL (Expected Failure) is expected to fail the test, which
means that test case works as we expected. IOW, XFAIL is same as
PASS. So make it green.

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