Expand WorkspaceTestAdapter tests #21020

These tests should be written to test the WorkspaceTestAdapter:
* - successful discovery but no data: delete everything from the test controller
* - successful discovery with error status: add error node to tree
* - single root: populate tree if there's no root node
* - single root: update tree if there's a root node
* - single root: delete tree if there are no tests in the test data
* - multiroot: update the correct folders