Add unittest for FolderTreeWidget.

This includes testing for:
- showing folders for the initial resources from the unittest env
- creating resources, they should appear in the tree
- creating a search folder, should create "Search / search123456"
- deleting that search folder, both should disappear, and things
shouldn't get corrupted (kitemmodels bug, fix pending, test disabled
until KF 5.26)

The dummykernel from src/filter/tests is moved up to autotests/ for sharing.