Convert test suite to thincloud-test (minitest)

Converts the test suite to use `thincloud-test`, swapping out RSpec for MiniTest and simplifying the test suite's dependency management.

@kbaum, this also adjusts a test of yours that was passing under RSpec, but broke under MiniTest which plays tests in a randomized order.  I'll add a line note to show where this came up.

/cc @phlipper, @elskwid - thanks for getting `thincloud-test` working outside of Rails, came in very handy here. :sparkling_heart: