[meta] Improve Project Browser's test coverage [#3316155]

Problem/Motivation

This module began as a Github project and was moved onto Drupal.org as of this commit.
Since then there have been 196 commits to the module:

63 with test coverage
100 with no test coverage needed (reverts, code cleanups, CSS-only changes)
11 with clear need for test coverage
22 with possible need for test coverage

Proposed resolution

Below are two lists. Each contain links to issues that were committed without test coverage.
New issues should be created for each, the old issues should not be reopened.

Needing coverage

#3283771: Moving to advanced filters using keyboard does not let the user know what it is for: Assert by aria-labelledby #3316740: Expand testAdvancedFiltering for AT
#3282732: Add UI text indicating that the current data may be stale or incomplete: Assert text #3316744: Write test to validate Drupal.org Mock API status message
fix placeholder text for Title - since it actually does a contains now: #3318442: Writing test coverage for fix placeholder text for Title.
#3238996: Provide menu link: Test added in #3315860: Change the position of the Browse tab in the local task order?
#3285879: Categories missing on List view: #3316454: Writing test coverage for categories missing on List view
#3224709: [API] Include API translation layer with mock/fixture implementation until d.o API is ready: Unit tests?
#3298575: URLs in body for projects are relative in the Mock plugin and creates 404s links and broken images: #3320295: Add test for relative to absolute URL conversion.
#3284880: Add copy-to-clipboard functionality for CLI snippets in modal: ?
#3240320: Filters for advanced / "all" search: Already has test coverage \Drupal\Tests\project_browser\FunctionalJavascript\ProjectBrowserUiTest::testAdvancedFiltering
#3277224: Pagination, then filtering broken - doesn't reset to page 1: #3316820: Writing tests for pagination, then filtering broken - doesn't reset to page 1
#3277264: Rework bottom pager component to match Drupal pagers: Already has test coverage \Drupal\Tests\project_browser\FunctionalJavascript\ProjectBrowserUiTest::testPaging

Possibly needing coverage

#3293424: Be consistent in using origin_url (and consider making it easier to include): Check changes to Search.svelte
#3240318: Streamline "teaser" (card) view of projects: Might have coverage already?
#3241358: Filter by multiple categories: Already has test coverage \Drupal\Tests\project_browser\FunctionalJavascript\ProjectBrowserUiTest::testCategoryFiltering
#3239477: Expose sort options: Already has test coverage \Drupal\Tests\project_browser\FunctionalJavascript\ProjectBrowserUiTest::testSortingCriteria
#3296250: machine_name field might not match the composer require namespace: Not sure
#3284463: Cache filter criteria in localStorage: Not sure if testable
#3285874: Display of Filters on Mobile: Not sure if testable
#3296264: Use drush pm:install in Download instructions: Probably not worth it
#3285880: Sort filter needs clarity: Probably not worth it
#3292019: Use puzzle-placeholder instead of "waiting..." for loading project images: Probably not worth it
#3284327: When sort status changes, convey the change to AT: Probably not worth it
#3282681: Skips heading level (h6 on XXX results): Probably not worth it
#3281216: Reuse core's AJAX throbber when things are loading: Probably not worth it
#3243028: Link without href tag in DownloadPopup.svelte: Probably not worth it
#3280176: Decouple front-end (Svelte) from "mockapi" to allow other plugins: There's got to be something testable here, right?
#3301577: Have a separate field/property for logo: ?
#3279663: Caching of results and validation of parameters: ?
#3279510: Reevaluate or remove DrupalOrgClient: ?
#3278972: Update List view design: ?
#3278352: Scaffolding and refactoring to use source plugins consistently: ?
#3274335: Make category names labels for the checkbox: ?
#3270210: Fix web path for icons and action button images in card and list display for sub directory and multilingual sites: ?

Remaining tasks

Evaluate the above lists and determine if they are testable
Strikethrough the ones which are not testable
Create new issues for the remaining issues, documenting here when they are made