Colorless when NO_COLOR env variable exists

In accordance to https://no-color.org/ standard

src/tests/main.cc

@@ -4,7 +4,7 @@ int main()
 {
 	using namespace boost::ut;
 
-	if (!isatty(STDOUT_FILENO)) {
+	if (!isatty(STDOUT_FILENO) || getenv("NO_COLOR") != nullptr) {
 		cfg<override> = options {
 			.colors = colors { .none = "", .pass = "", .fail = "" }
 		};