Add unit tests for UndoRedo

[TheSofox]

Added Unit Test for the UndoRedo system.

This one tests basic functionality and should pass fine.

I'm getting a warning about "ObjectDB instances leaked at exit", but I get the exact same error when running "test_object" (which I used for help with some of the code I wrote) and I double checked my memory allocation so I'm not sure where this is coming from.

Edit: Mentioning #43440 as it's a unit test.

[AThousandShips]

You seem to have missed some of your code, or was this meant to be a draft?

[TheSofox]

Apologies. Pushed correct version.

[AThousandShips]

Still not right, you've not included the actual test file, did you forget to git add them?

Resolved

[TheSofox]

I do want to mention that there are other UndoRedo tests I want to write but was waiting for this PR to be accepted first so I could build on it.

I also want to write tests for MERGE_ALL function of 'UndoRedo' once my PR to fix it has been merged: #85390

[TheSofox]

This PR has served its purpose. When adding Unit Tests to #85390 to demonstrate I'd fixed the problem, I transferred the Unit Tests from this PR to that one. Thanks @AThousandShips for ensuring this code was in good condition. The code is now in the engine so this specific PR no longer has a purpose.