Ref: #1257

Summary:

• Fixed some bugs in the production code (unit tests were correctly failing for me).
• Fixed a couple of tests.
• Moved SqlQueryParserTest to unit tests.
• WpConfigEditor received a small cleanup and doc update.
• Some obsolete utils like WpAutomationRunner.sample.php or testing Gulp tasks were removed.
• The tests folder now has a README pointing to Dev-Setup#testing.
• Dockerfiles versioning to avoid surprises with stale images.
• Cleaned up testing docker-compose.yml
• Improved testing instructions in Dev-Setup.md.

After this PR, all test groups (Unit, End2End, Worklow, ...) pass when run separately. Two most obvious issues are now:

1. Tests fail when run together, there is probably some interference.
2. End2end tests take vary long to initialize (6 minutes on my PC). This is proportional to how many tests are actually going to be run and feels like a new bug to me; I don't remember test initialization being that slow on local setup.