Use default client instead of create a new client when running tests

[smartinm]

When I run the tests, I get following errors after a while:

Mongo::Error::NoServerAvailable:         Mongo::Error::NoServerAvailable: No server is available matching preference: #<Mongo::ServerSelector::Primary:0x007fe8fa5593e0 @options={"mode"=>:primary, "database"=>"admin"}, @tag_sets=[], @server_selection_timeout=30>
D, [2015-11-17T18:00:13.977278 #32569] DEBUG -- : MONGODB | Adding 127.0.0.1:27017 to the cluster.
D, [2015-11-17T18:00:13.980724 #32569] DEBUG -- : MONGODB | IOError
D, [2015-11-17T18:00:14.480547 #32569] DEBUG -- : MONGODB | IOError

I think the problem is because a new client is created every time in teardown.

This PR modifies BaseTestCase to use default client instead of create a new client when running tests.