Consolidate tests into clear unit & integration folders

[pushfoo]

Enhancement request:

What should be added/changed?

Per discord discussion, the following changes should be made as time is available:

• unit and unit2 should be consolidated
• add an integration folder if necessary
• move the tests in the gui, tiled, tilemap_tests, etc folders into the appropriate consolidated folders

This would make the test hierarchy more legible to everyone, including less experienced contributors.

[einarf]

#1614