Refactor tests for i04 grid detect the XRC plan entry

The tests in `test_i04_grid_detect_then_xray_centre_plan.py` are ugly mainly due to how many repeated patches we are using for each individual test. We should tidy this up by making sure only the essential patches are used for each of the tests, and grouping all the tests which require a patch by using a test class, (eg see `TestFlyscanXrayCentrePlan`)

## Acceptance Criteria
- `test_i04_grid_detect_then_xray_centre_plan.py` doesn't contain tests with repeated patches, and we confirm that only the required patches are used for each test.