Refactor tests to use generated keys

Wrapping a key generation function lets us expand things in the future,
such as providing an option to delete keys before they are used, or
allowing separate prefixes so tests can execute in parallel.

Change-Id: Id3b8362fde9720988f2f3816c18f91048306f342
Reviewed-on: http://review.couchbase.org/26316
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>