Fix preview handlers failing unit tests in CI (#1856)

* Enable Unit tests for PreviewPane Test Assemblies in CI

* Added Preview handlers unit tests project back

* throw for Svg to debug tests

* Removed failing test

* Removed throw statement

* Added MD unitests in yml

* Run ui tests on STA thread

* Throw svg to debug failing tests

* Updated test adaptor

* Removed failing test to validate CI

* throw svg failing test with full stack trace

* Check if mock stream is causing the issue

* narrow down the failing tests code - 1

* narrow down the failing tests code -2

* dispose all controls

* Restore SVg preview control

* Update test adaptor and dependency to latest

.pipelines/ci/templates/build-powertoys-steps.yml

@@ -33,3 +33,16 @@ steps:
     msbuildArgs: ${{ parameters.additionalBuildArguments }}
     clean: true
     maximumCpuCount: true
+
+- task: VSTest@2
+  inputs:
+    platform: '$(BuildPlatform)'
+    configuration: '$(BuildConfiguration)'
+    testSelector: 'testAssemblies'
+    testAssemblyVer2: |
+      **\PreviewPaneUnitTests.dll
+      **\UnitTests-SvgPreviewHandler.dll
+      **\UnitTests-PreviewHandlerCommon.dll
+      **\powerpreviewTest.dll
+      !**\*TestAdapter.dll
+      !**\obj\**