static only(item) {
		if (typeof item === "string") {
			return { code: item, only: true };
		}

		return { ...item, only: true };
	}
Adds the `only` property to a test to run it in isolation.
@param {string | ValidTestCase | InvalidTestCase} item A single test to run by itself.
@returns {ValidTestCase | InvalidTestCase} The test with `only` set.