kunit: tool: Do not colorize output when redirected

Filling log files with color codes makes diffs and other comparisons
difficult. Only emit vt100 codes when the stdout is a TTY.

Cc: Brendan Higgins <brendanhiggins@google.com>
Cc: linux-kselftest@vger.kernel.org
Cc: kunit-dev@googlegroups.com
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>