chore(null): null safety fixes for assorted test helpers

[dbjorge]

Description of changes

This PR fixes all yarn null:find issues in common test helpers, plus all files yarn null:autoadd picked up.

New yarn null:progress:

Web strict-null progress

47% complete (635/1362 non-test files)

Contribute at #2869. Last update: Tue Oct 13 2020

Pull request checklist

• Addresses an existing issue: Codebase should use strict null checks #2869
• Ran yarn fastpass
• Added/updated relevant unit test(s) (and ran yarn test)
• Verified code coverage for the changes made. Check coverage report at: <rootDir>/test-results/unit/coverage
• PR title AND final merge commit title both start with a semantic tag (fix:, chore:, feat(feature-name):, refactor:). See CONTRIBUTING.md.
• [n/a] (UI changes only) Added screenshots/GIFs to description above
• [n/a] (UI changes only) Verified usability with NVDA/JAWS

[dbjorge]

This is the most interesting fix. The contract for Playwright's waitForSelector is that it will throw rather than returning null if it can't find an element, except that if you explicitly pass options like {state: 'detached'}, then it will return null in the success path. So waitForSelectorInShadowRoot is allowed to return null (since someone could pass through that option), but waitForShadowRoot gets to assume that waitForSelector is not returning null (since it's passing state: 'attached' always)

[DaveTryon]

Glad to see these going away--it was not a good code smell to have to change a bunch of downstream test classes when adding a field to the properties