GD-135: Allow to use AssertSignal on RefCounted Objects (#136)

# Why
see #135

# What
- Added Node instance check before register the `UnregisterEmitter` on
`TreeExiting` signals
- Added custom cleanup method to the GodotSignalCollector to enshure all
registered emitters are unregistered after the test execution
- Covered by new test `EmitSignalOnNoneNodeObjects`
- Some code cleanup/formattings
- Update Example Project to latest API and Test Adapter
- Increase API version to 4.3.1

Author: MikeSchulze

PackageVersions.props

@@ -5,7 +5,7 @@
     <MicrosoftSdkVersion>17.9.0</MicrosoftSdkVersion>
     <MicrosoftCodeAnalysisCSharpVersion>4.9.2</MicrosoftCodeAnalysisCSharpVersion>
     <MoqVersion>4.18.4</MoqVersion>
-    <GdUnitAPIVersion>4.3.0</GdUnitAPIVersion>
+    <GdUnitAPIVersion>4.3.1</GdUnitAPIVersion>
     <GdUnitTestAdapterVersion>2.0.0</GdUnitTestAdapterVersion>
   </PropertyGroup>
 </Project>

api/ReleaseNotes.txt

@@ -1,3 +1,10 @@
+v4.3.1
+
+Bug Fixes:
+* GD-135: Cannot test signals on non-node classes
+
+-------------------------------------------------------------------------------------------------------
+
 v4.3.0
 
 Improvements: