This patch makes the following changes:
1. Converts the xpcshell tests test_clearPopulatedForm.js and test_previewFormFields.js to browser tests.
This change is necessary because the logic is moved from the child to the parent, requiring integration
tests instead.
2. Replaces the custom popup listener in FormAutofillChild with `BrowserTestUtils.waitForPopupEvent`
3. Updates add_heuristic_tests to align with the new architecture.
4. Updates unit tests that use the 'active' section because there is no "active" section anymore.
Differential Revision: https://phabricator.services.mozilla.com/D211554
f4a9818b2f