function subsetTestByKey(key, testFunc, ...args) {
    if (collectKeys) {
      if (collectCounts && key in keys) {
        keys[key]++;
      } else {
        keys[key] = 1;
      }
    }
    if (shouldRunSubTest(key)) {
      return testFunc(...args);
    }
    return null;
  }
Only test a subset of tests with `?include=Foo` or `?exclude=Foo` in the URL.
Can be used together with `<meta name="variant" content="...">`
Sample usage:
for (const test of tests) {
  subsetTestByKey("Foo", async_test, test.fn, test.name);
}