Fix test_grid_overlay OAV system test

This test is currently failing on `main` but is skipped so it's not notice. It at least needs to actually connect the PVs with `async with init_devices(connect=True):` but even then there is a timeout.

FYI @DiamondLightSource/developers-mx-daq, I can try and fix this if I get a bit of time but it's not used outside MX AFAIK

## Acceptance Criteria
* The test is fixed or removed

Start by locating the skipped test_grid_overlay OAV system test and inspect how it initializes its PVs. Try the mentioned async with init_devices(connect=True) setup, then investigate the timeout while running the test. Done means the test passes reliably or is removed, as specified by the acceptance criteria.

Written by the indexing model from the issue text.