Fix NUnit tests by migrating to use Unity Test Frameworks. (#646)

- Change NUnit tests to utilize Unity Test Frameworks
- Add new script to enable dlls through Version Handler. This script should be removed before tests, or it will exit the Unity once Version Handler update is done.
- Add required UPM packages for Unity Test Frameworks
- Move all NUnit test code to the proper folders and add proper .asmdef, instead of using .csproj

Author: chkuang-g

.github/workflows/test.yaml

@@ -132,11 +132,10 @@ jobs:
         shell: bash
         timeout-minutes: 60
         run: |
-          # DISABLE: NUnit test is currently broken from Unity 2019
           ./gradlew test -q \
                     -PINTERACTIVE_MODE_TESTS_ENABLED=0 \
                     -PINCLUDE_TEST_TYPES="${{ needs.check_and_prepare.outputs.include_test_types }}" \
-                    -PEXCLUDE_TEST_TYPES="NUnit,${{ needs.check_and_prepare.outputs.exclude_test_types }}" \
+                    -PEXCLUDE_TEST_TYPES="${{ needs.check_and_prepare.outputs.exclude_test_types }}" \
                     -PINCLUDE_TEST_MODULES="${{ needs.check_and_prepare.outputs.include_test_modules }}" \
                     -PEXCLUDE_TEST_MODULES="${{ needs.check_and_prepare.outputs.exclude_test_modules }}" \
                     -PEXCLUDE_TESTS="${{ needs.check_and_prepare.outputs.exclude_tests }}"