Add simple CI script to ensure that all pgregress tests are part of the schedule

### Description

#563 introduced a test that was not part of the schedule. We should add a simple CI script that detects such dumb mistakes.

Start by inspecting the existing pgregress tests, their schedule, and the repository's CI configuration. Add a CI check that detects tests missing from the schedule, then run the relevant pgregress and CI checks to confirm every test is covered.

Written by the indexing model from the issue text.