add scripts for C# windows tests

csharp/buildall.bat

+@rem Builds Google.Protobuf and runs the tests
+
+dotnet build src/Google.Protobuf.sln || goto :error
+
+echo Running tests.
+
+dotnet test -f netcoreapp1.0 src/Google.Protobuf.Test/Google.Protobuf.Test.csproj || goto :error
+
+goto :EOF
+
+:error
+echo Failed!
+exit /b %errorlevel%

kokoro/windows/csharp/build.bat

+@rem enter repo root
+cd /d %~dp0\..\..\..
+
+cd csharp
+
+@rem Install dotnet SDK
+powershell -File install_dotnet_sdk.ps1
+set PATH=%LOCALAPPDATA%\Microsoft\dotnet;%PATH%
+
+call buildall.bat

kokoro/windows/csharp/continuous.cfg

+# Config file for running tests in Kokoro
+
+# Location of the build script in repository
+build_file: "protobuf/kokoro/windows/csharp/build.bat"
+timeout_mins: 1440