WebDriver executor ends each `testdriver` action with the `testharness.js` window selected

[jonathan-j-lee]

See #44688 for full background.

The WebDriver implementation of testharness.js must switch to the testharness.js browsing context to run testharness_webdriver_resume.js. In a tabbed browser with popups in the same OS window as the testharness.js context, this will cause the popups to be hidden between test_driver.* calls, which may make some popup visibility interactions untestable.

CC @khushalsagar @jgraham

[jgraham]

Yes, solving this properly probably requires WebDriver-BiDi support in testdriver, so we can message navigables without actually focusing them.