chore: avoid silent test failures by flushing Promises after each test

[dbjorge]

Details

This PR adds a new setup step to each of our test suites which forces Jest to flush all Promises in a globally applied afterEach fixture.

This forces Jest to flush any pending resolved Promises that a test might have forgotten to handle before Jest considers that test to be completed. That way, if a test leaks a Promise, the resulting error message gets logged as a failure in the responsible test and includes a nice, Jest-formatted stack, instead of test "passing" and the error as a Node UnhandledProjmiseRejectionWarning with no stack or test context.

This is intended to work in tandem with #5061, allowing it to go in as a last-resort while mitigating the poor logging that we get if it's actually triggered (by avoiding having it need to trigger in the common case)

Motivation

Avoids silent test failures (eg, the one that currently exists in injector-controller.test.ts)

Context

jestjs/jest#10784 (comment)

Pull request checklist

• [n/a] Addresses an existing issue: #0000
• 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