Add .netcoreapp 3.1 dependencie for tests

Author: glucaci

.github/workflows/dotnet.yml

@@ -16,7 +16,9 @@ jobs:
     - name: Setup .NET
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: 6.0.*
+        dotnet-version: | 
+          3.1.x
+          6.0.x
     - name: Restore dependencies
       run: dotnet restore
     - name: Build