Write catch2 unit tests for filling holes in the bitmap.

Original ticket description:

At least the following tests cases should be covered. Please consider during implementation whether further cases are required.

️Removing a hole:

• Entirely inside an extent
• Partially overlapping an extent on the left
• Partially overlapping an extent on the right
• Fully overlapping an extent
• Exactly matching an extent
• Overlapping multiple extents
• Not overlapping an extent
• Fully overlapping the final extent in the list. The extent list becomes NULL

This may uncover bugs that can also be fixed under this ticket. Feel free to break work into separate tickets if the work becomes too large.