ci: Partial setup for running unit tests in new CI infrastructure.

This PR includes:
- The Windows Dockerfile that will be used for Windows builds.
- A script that runs unit tests.

The rest of the setup is internal.

.kokoro/gcp_windows_docker/Dockerfile

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+# We use the .NET 6 Windows Server Core 2019 official image as starter.
+# Kokoro supports up to Windows Server 2019 only.
+# We build in .NET 6
+FROM mcr.microsoft.com/dotnet/sdk:6.0-windowsservercore-ltsc2019
+
+# Install extra .NET target frameworks that we test in.
+# https://github.com/dotnet/dotnet-docker/blob/main/documentation/scenarios/installing-dotnet.md#installing-from-dotnet-install-script
+# Setup Powershell
+RUN powershell -Command $ErrorActionPreference = 'Stop';
+RUN powershell -Command $ProgressPreference = 'SilentlyContinue';
+# Download dotnet-install.ps1
+RUN powershell -Command Invoke-WebRequest -UseBasicParsing -Uri https://dot.net/v1/dotnet-install.ps1 -OutFile dotnet-install.ps1;
+# Install .NET Core 3.1
+RUN powershell -Command ./dotnet-install.ps1 -InstallDir '/Program Files/dotnet' -Channel 3.1;
+# Delete dotnet-install.ps1
+RUN powershell -Command Remove-Item -Force dotnet-install.ps1;
+
+# Install Chocolatey
+ENV ChocolateyUseWindowsCompression false
+RUN powershell -NoProfile -ExecutionPolicy Bypass -Command "iex ((New-Object System.Net.WebClient).DownloadString('https://chocolatey.org/install.ps1'))" && SET "PATH=%PATH%;%ALLUSERSPROFILE%\chocolatey\bin"
+
+# Install Chocolatey packages
+RUN choco install git.install -y
+
+# Default to PowerShell.
+# This is what Kokoro owned Docker images do. We'll do the same at least for now.
+CMD ["powershell.exe", "-NoLogo", "-ExecutionPolicy", "Bypass"]

RunUnitTests.sh

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+#!/bin/bash
+
+set -e
+
+dotnet build Src/Support/GoogleApisClient.sln
+dotnet test --no-restore Src/Support/Google.Apis.Auth.Tests/Google.Apis.Auth.Tests.csproj
+dotnet test --no-restore Src/Support/Google.Apis.Tests/Google.Apis.Tests.csproj