CSHARP-1109: Added MongoDB.Driver.Core.TestHelpers to house test related helper classes that are shared by multiple test projects.
CSHARP-1109: Remove the new internal constructor from MongoClient and change the Configuration static constructor back to using the original MongoClient constructor. This way our unit tests will be using the same code path as our users.
Ensure that at the end of a test run all test databases have been dropped.