To facilitate code sharing in LINQ3 tests add a `Linq3Client` property to `DriverTestConfiguration`. This will return a client that is the same as the one returned by `DriverTestConfiguration.Client` except that it will be configured to use the `V3` LINQ provider.

Also add `AssertStages` and `Translate` helper methods in a `Linq3TestHelpers` class that can be called from LINQ3 tests.