Save gitlab.com sign-in page acessibility report as test artifact

This is a small MR that adds a line to the test script to write an HTML report to a file. We can then persist this artifact in the check-executables CI job and folks can look at the actual output when we run test pipelines.

I set up this branch off of the original feature branch for !4 (merged) and used it to check the file output there. Since I found it helpful in doing testing/QA for this Docker image, I figured we should leave it in our test pipelines.

I also changed the test to use our sign in page on dot-com instead of the team page because of the long loading time on the team page.

This also sets up the test structure for shipping a test with sample JSON artifacts: !5 (merged)