(PUP-3030) Add Uniquefile tests, ported from Ruby source

This commit adds additional tests for the Uniquefile class.  The
new tests are ported from the Ruby 1.9.3 source code for the
built-in `Tempfile` class, since that is what our `Uniquefile`
implementation is based on.

See: https://github.com/ruby/ruby/blob/v1_9_3_547/test/test_tempfile.rb