Migrate conditional tests to gate pragma (#18585)

* Migrate conditional tests to gate pragma

I searched through the codebase for this pattern:

```js
describe('test suite', () => {
  if (!__EXPERIMENTAL__) { // or some other condition
    test("empty test so Jest doesn't complain", () => {});
    return;
  }

  // Unless we're in experimental mode, none of the tests in this block
  // will run.
})
```

and converted them to the `@gate` pragma instead.

The reason this pattern isn't preferred is because you end up disabling
more tests than you need to.

* Add flag for www release channels

Using a heuristic where I check a flag that is known to only be enabled
in www. I left a TODO to instead set the release channel explicitly in
each test config.