GD-635: Rework on test case discovery and test explorer comunication

[MikeSchulze]

Describe What

• discovery test cases as a simple struct and not as a Godot Node
• send the discovery by a sink to the test explorer
• test explorer
  • collect tests and build the test tree by the new test case structure
  • using guid to address the test case unique
  • using the full nest name to build the tree
  • using sink to add test cases to the tree
  • using events to add dynamic created test cases to the tree
• test status bar
  • using sink to count tests
  • using events to add dynamic created test cases to count

Describe Why
We need to adapt the changes of the GdUnit4Net API to enable the GdUnit4 API to connect.
The current handling of discovery test cases are very different and we need a common approach.

Dev hints
see discovery works on the gdunit4.test.adapter