Add .NET test to github workflows (#6982)

.github/workflows/build.yml

@@ -80,6 +80,34 @@ jobs:
     - name: test
       run: Release\flattests.exe
 
+  build-dotnet-windows:
+    name: Build .NET Windows
+    runs-on: windows-latest
+    strategy:
+      matrix:
+        configuration: [
+          '',
+          '-p:UnsafeByteBuffer=true', 
+          # Fails two tests currently.
+          #'-p:EnableSpanT=true,UnsafeByteBuffer=true'
+          ]
+    steps:
+    - uses: actions/checkout@v1
+    - name: Setup .NET Core SDK
+      uses: actions/setup-dotnet@v1.9.0
+      with: 
+        dotnet-version: '3.1.x'
+    - name: Build
+      run: |
+        cd tests\FlatBuffers.Test
+        dotnet new sln --force --name FlatBuffers.Core.Test
+        dotnet sln FlatBuffers.Core.Test.sln add FlatBuffers.Core.Test.csproj
+        dotnet build -c Release ${{matrix.configuration}} -o out FlatBuffers.Core.Test.sln
+    - name: Run
+      run: |
+        cd tests\FlatBuffers.Test
+        out\FlatBuffers.Core.Test.exe
+
   build-mac:
     name: Build Mac
     runs-on: macos-latest