selftests/ftrace: Use colored output when available

If test is being directly executed (with stdout opened on the
terminal) and the terminal capabilities indicate enough
colors, then use the existing scheme of green, red, and blue
to show when tests pass, fail or end in a different way.

When running the tests redirecting the stdout, for instance,
to a file, then colors are not shown, thus producing a more
readable output.

Signed-off-by: Daniel Díaz <daniel.diaz@linaro.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>