Add tests for the layer tree

[0HyperCube]

Test cases:

• Layers can be selected with shift and ctrl in the layer tree (Add layer tree tests for layer selection #2577)
• Layers can be rearranged (Add layer tree tests for layer arrangement #2578)
• Moving a folder into itself doesn't crash (Add layer tree tests for folder self movement checking #2579)
• Moving a folder also moves children children (Add layer tree tests for folder movement checking into child #2580)
• Moving with transforms, see: #2212 (Add layer tree tests for moving layers with transform #2585)

[Keavon]

Closed by #2577, #2578, #2579, #2580, and #2585.

[rahat2134]

@0HyperCube @Keavon , This issue is completed with the referenced PR's. Thanks!