Report scenarios that are filtered out by unmet requirements #392

Scenarios that cannot run in the current environment are filtered out through tags (@require-php-*, @require-wp-*, @require-mysql, @require-extension-*, OS tags and so on). This happens silently: nothing in the output says which scenarios were left out or why, so a green run can hide missing coverage.

Steps:

1. Before the run starts, list every scenario that is filtered out together with the requirement that is not met. On CI, add the list to the job summary.
2. Add a strict mode that stops before the run when a requirement is not met, instead of filtering. CI and the container runner can use it, because there a missing requirement is a problem with the environment and should be fixed rather than skipped.