XunitTraitsDiscoverers: add reference to System.Threading

This allows the test project to be built with Mono's xbuild.
It's the same change as in https://github.com/dotnet/corefx/pull/288.
Add reference from RegularExpressions to XunitTraitsDiscoverers

This is required to make the solution self-contained. It also follows
other solutions that already reference XunitTraitsDiscoverers, such as

* System.Linq.Parallel.sln
* System.Numerics.Vectors.sln
* System.Reflection.Metadata.sln
* System.Text.RegularExpressions.sln
* System.Threading.Tasks.Dataflow.sln
* System.Xml.sln
Ensure building in Visual Studio works without having to run build.cmd

Currently, you need to run build.cmd once before you can open and build
any of the solutions in Visual Studio. The reason is that the project
targets have dependencies on NuGet packages that aren't expressed via
a packages.config. So while the Visual Studio NuGet is able to restore
NuGet packages, it can't restore those because it doesn't know about
the build-tools dependency.

I've fixed that by adding a top-level .nuget folder which is recognized
by NuGet as global dependencies. The only reason I've added this folder
to each solution is because the NuGet client will add it automatically
to the solution on first build. So to avoid future noise, I've added
the file to each solution already.

Also, in order to make the solutions build immediately, we can't rely
on the build tools being copied to $(OutDir)tools, which is what the
current build process does. Instead, I've changed the build to directly
run from the package location.
Merge pull request #324 from terrajobst/vs-build

Ensure building in Visual Studio works without having to run build.cmd
Merge pull request #328 from akoeplinger/add-threading-reference

XunitTraitsDiscoverers: add reference to System.Threading