[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.
Add additional web file formats for Content

Added:
  - .less
  - fonts
  - .svg
    - Included as its own entry since it's not strictly an image format
[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.
[Core] Remove duplicate CredentialStore singleton
[Mac] Handle keychain persistence in MacProxyCredentialProvider
[Mac] Don't make duplicate/concurrent credential prompts
[Mac] Read system proxy credentials

Fixes BXC 19828 - User is prompted to re-enter proxy credentials
in XS, while credentials are already entered in system preferences
on Mac
[TextEditor] Fixed reserved keyowords in F# language.
Fixed 'Bug 19811 - F# reserved OCAML token asr fails to highlight'
Fixed 'Bug 19812 - F# reserved OCAML token land fails to highlight'
Fixed 'Bug 19813 - F# reserved OCAML token lor fails to highlight'
Fixed 'Bug 19814 - F# reserved OCAML token lsl fails to highlight'
Fixed 'Bug 19815 - F# reserved OCAML token lsr fails to highlight'
Fixed 'Bug 19816 - F# reserved OCAML token lxor fails to highlight'
Fixed 'Bug 19817 - F# reserved OCAML token mod fails to highlight'
[TextEditor] Fixes some F# highlighting issues.
Fixed 'Bug 19809 - F# not keyword fails to highlight'
Fixed 'Bug 19810 - F# select keyword fails to highlight'
Fixed 'Bug 19808 - F# let keyword highlights only after entering a
space after word'
Also fixed issues with keywords that ended with '!'. From the
highlighting perspective F# is a complex language.
[NuGet] Add tests for BackgroundPackageActionRunner.
Immediate window CodeCompletion appeared above typing text fixing Bug 14578
Merge pull request #553 from DavidKarlas/immediateWindowCodeCompletion

Immediate window CodeCompletion appeared above typing text fixing Bug 14578
[Debugger] Disable more SDB tests when running with an old mono