Rename test class to be accurate

It is not testing Database, it's testing MongoDriver.
Use MongoClient if blank replica set is provided

The replica set name is provided with the MONGO_REPLICA_SET environment
variable. If this is set to an empty string the MongoClient will be used
rather than the MongoReplicaSetClient.

For example:
MONGO_REPLICA_SET='' ./run_tests.sh

This is required for running in environments that do not provide a
replica set, such as travis.
Set MONGO_REPLICA_SET to empty for travis builds

Travis provides a mongodb service that is not part of a replica set so
using a MongoRecplicaSetClient does not work.
Merge pull request #235 from alphagov/set-travis-mongo-rs

Fall back to MongoClient