[NuGet] Add first group of unit tests.

Port across most of the existing NuGet addin unit tests from
SharpDevelop.

Unit tests that are currently missing are those that use the Project
or Solution classes.
[NuGet] Add more unit tests.

Port remaining unit tests from SharpDevelop.
[NuGet] Fix unit tests on Mac

Use native file paths on non-Windows operating systems in unit
tests.
[NuGet] Fix unused variable compiler warnings.
[NuGet] Fix invalid cast exception.

DotNetProject wrapper class introduced with unit tests was
incorrectly being cast to the wrong type.
[NuGet] Fix user's NuGet.Config being modified by unit tests.
[NuGet] Add install package action tests.

Add tests for generating warnings for packages with licenses
or packages that have PowerShell scripts.
[NuGet] Add unit tests for XDT fixes.

Add tests to check the changes made to the Xml Document Transformation
library and verify it works on Mono when running Remove or
SetAttributes transforms.

This should catch if the wrong Microsoft.Web.XmlTransform.dll is used.

These tests should be removed once the XDT library is being built from
source code along with NuGet. The XDT library contains its own set of
unit tests that can be run on Mono which check these transforms are
working.
[NuGet] Check ProjectTypeGuids are read from project.

When installing a NuGet package the project's ProjectTypeGuid is used
to determine if a project is a web project. This determines whether
the web.config or app.config is affected by any transforms included
in the NuGet package.
[NuGet] Add tests for BackgroundPackageActionRunner.
[TextEditor] Fixed typo
Merge branch 'master' into nuget-tests
[NuGet] Fix unit tests after merge from master.

Unit tests were incorrectly setting up ProjectReferences in a
DotNetProject. They were relying on incorrect behaviour of the
ProjectReference class. The ProjectReference class was changed so the
Reference property is the assembly name and not the assembly filename.
[NuGet] Fix compiler warning.
Merge branch 'nuget-tests'