Move test projects to SDK-based .csproj

[joelverhagen]

This will make transitive project references work much better. This would help with a repository merge since a package reference switching to a project reference will then still retain transitive dependency information. In legacy .csproj, project references are not transitive so without this change we would need to lift a lot of project dependencies into consuming projects when, say, NuGetGallery.Core switches from package reference to project reference.

This work also has the effect of modernizing our code base a bit. It theoretically makes a .NET Core move easier too.

This was done using the latest version of the .NET upgrade-assistant CLI tool with the following options:

upgrade-assistant upgrade {csproj path} --operation sdkstyle --non-interactive.

Unit tests will be run using dotnet test instead of xunit.console.runner. This is a modernization step. Functional tests still use xunit and vstest due to compatibility with old web test framework we use.

This does most of the hard work. After this, I did manual clean-up steps:

1. Add SdkProjects.props import
2. Add <IsTestProject>true</IsTestProject> to functional test .csproj. Xunit test projects have this automatically.
3. Remove unnecessary package, project, and assembly references.
4. Delete AssemblyInfo.cs unless there was Xunit parallelization attributes or custom metadata
5. Delete App.config in most cases (only leave minimal necessary things like EF directives)
6. Rename BuildTests.ps1 to BuildGalleryFunctionalTests.ps1 to ease repository merge with NuGet.Jobs
7. Move functional test .sln to be in the root so it's easier to find
8. Move to latest lang version
9. Move to latest build tools
10. Switch PublishTestResults CI step to the latest version and change format to VSTest (dotnet test) so test results are found