[CI] Add Python and C++ tests for Windows GPU target

[hcho3]

Follow-up to #4463. At minimum, we want to run Python and C++ tests. In particular, we want to ensure that the generated Python wheel is broadly compatible, e.g. on following platform combinations:

1. Windows Server 2008 R2 without GPU
2. Windows Server 2012 R2 with GPU + CUDA 9.0
3. Windows Server 2016 with GPU + CUDA 10.0
4. Windows Server 2019 with GPU + CUDA 10.1

[hcho3]

MSBuild sometimes fails spuriously with error:

error MSB4175: The task factory "XamlTaskFactory" could not be loaded from the assembly "Microsoft.Build.Tasks.v4.0, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a". Could not find file 'C:\Users\Administrator\AppData\Local\Temp\4irrjman\4irrjman.dll' [C:\Jenkins\workspace\xgboost-win64_PR-4469\build\src\objxgboost.vcxproj]

(https://xgboost-ci.net/blue/organizations/jenkins/xgboost-win64/detail/PR-4469/10/pipeline#step-43-log-411)

In dotnet/msbuild#899 (comment), it is suggested to use /nodeReuse:false. I will give it a try.

[hcho3]

Looks like /nodeReuse:false removed the MSB4175 error. Nice.

[hcho3]

@rongou @canonizer @sriramch Feel free to suggest additional ways to test GPU algorithms on the Windows platform.

[hcho3]

@RAMitchell Can you review?