Create a new user on the server with a generated (and hopefully unique) name.
The username will be of the form testuser-COUNT-RANDOM where COUNT is numbered sequentially from 1, and RANDOM is
a pseudo-randomly generated for each test. The COUNT is reset at the start of each test. If this method is
invoked multiple from the same test, then COUNT will be incremented by one each time, but RANDOM will stay the same.
This gives a pattern that looks like test1, test2, test3 etc, whilst still ensuring uniqueness the avoid clashes
between tests not performing teardown 100% reliably.