Draft: Experiment with remote tests

Switch tests to use the tests in the doclib repo.

• Remove local tests
• Add CI steps to clone and install dependencies in doclib repo
• Update scheduled CI step to check links and anchors (currently running every Saturday)
• On each MR, run tests on changed files
• For tests that use Mocha, generate Junit reports and post them to the MR
• Do not block on failed tests (for now)

These tests will now run on each MR:

• Check links to images in changed MD/MDx files
• Check to see if any images in the static folder are no longer used
• Check to see if the changed files reference any dependencies that are not noted in the front matter
• Check external links in the changed files

Problem: The workflow fails if there are no changed MD/MDX files.

Steps for later MRs:

• Better reporting; clarify names so the test results are clearer
• Move other tests to Junit, or work out reoprting for them

I've left some test failures in this MR so you can see what they look like:

You can click through to the full report: