[wptrunner] Add `infrastructure/expected-fail/` test for user prompts

[jonathan-j-lee]

This test exercises the handling of a testharness.js test that opens user prompts like window.alert(...). For WebDriver-controlled browsers that default to "dismiss and notify", this should raise a harness error in the executor's testdriver loop.

Fix a related issue wptrunner-side where creating a user prompt can cause testharness_webdriver_resume.js to resolve with null/None, which would then fail to unpack in:

https://github.com/web-platform-tests/wpt/blob/da25e93244e17fa16f245a955af8e0638230acab/tools/wptrunner/wptrunner/executors/base.py#L735

[WeizhongX]

@jgraham, can you also take a look? Thanks!