Remove _fake_grid_detection and use real grid detection instead

[DominicOram]

In a similar issue to #479 we have some tests that are running against _fake_grid_detection. This means if we update the grid detection prod code we need to update this too and so they will inevitably get out of sync. Instead all our tests should be run against the real grid detection prod code.

Acceptance Criteria

• _fake_grid_detection is removed
• Tests that used it instead use production grid detection code