refactor: add util type for asserting on global test props

[jcfranco]

Related Issue: N/A

Summary

Introduces helper to tack on typed global props for testing purposes.

Here's a usage example:

it("does something", async () => {
  // ... page setup

  type TestWindow = GlobalTestProps<{
    someTestPropForLater: boolean;
  }>;

  await page.evaluate(() => {
      //...browser-context code
      (window as TestWindow).someTestPropForLater = true;
  });

  //... some test steps

  const result = await page.evaluate(() => (window as TestWindow).someTestPropForLater);

  expect(result).toBe(true);
});

Pulls util from #2777 to make the PR a bit leaner.

[driskull]

👍