Fixing find_and_modify RS test to include a query

This resolves a failure on jenkins:
https://jenkins.10gen.com/job/mongo-ruby-driver-1.x-stable/245/mongodb_server=stable-release,os_arch=linux64,ruby_language_version=2.1.0,test_type=replica-set/console

This test is more precise if a unique value is used for 'a' and then used to query and update the document. Otherwise there is the possibility of docs being left in the collection from other tests. The find_and_modify command would update one of these docs and then the find_one could return a different doc than the one that was updated.

Cleanup after other tests needs to be investigated as well...