Update tests.yml

Author: coderookie1994

.github/workflows/tests.yml

@@ -16,7 +16,7 @@ jobs:
     - name: Setup .NET
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: 5.0.x
+        dotnet-version: 3.1.x
     - name: Restore dependencies
       run: |
         cd tests