Upgrade tests to use Xunit 2.0

- Changed tests to use Xunit 2.0.
- Whenever we use [OuterLoop] or [ActiveIssue(number)] I add reference to XunitTraitsDiscoverers
- Made sure everything compiles cleanly from VS and build.cmd

The only project which isn't using XunitTraitsDiscoverers is ImmutableCollections - it is using different Profile so I couldn't reference it.