This change brings a very minimal set of tests and make
build fail, if we cannot load some PS assamblies on FullCLR
We should increase test coverage for FullCLR in future.
Commands.Utility now needs Microsoft.CodeAnalysis.CSharp, which doesn't
explicitly target `dnxcore50`, so now Commands.Utility and the packages
which depend on it now much `import` the Portable Windows framework.
This also required adding the aspnetvnext feed.
System.Management.Automation now requires System.Diagnostics.StackTrace.
- Pester source code moved to `test/Pester`, deleted `ext-src`.
- Pester tests (.ps1 files) moved to `test/powershell`
- xUnit tests (.cs files) moved to `test/csharp`
- Third-party script test moved to `test/shebang`