Added .NET Core SDK version and unit tests running

Author: xperiandri

.github/workflows/build.yml

@@ -13,9 +13,18 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
+
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: 2.1.607
+
     - name: Build with dotnet
       run: |
         DOTNET_CLI_TELEMETRY_OPTOUT=1
         dotnet build --nologo --configuration Release
+
+    - name: Run Unit Tests
+      run: |
+        cd StringBuilderExtensionsTests
+        dotnet run -c Release --no-build