The order-dependent failure has been been commented in code.
After manually bisecting the test suite, I was able to determine that
the template cascade tests were leaving the `@@template_priority_path`
set to an overridden value in tests run afterward.
`Test::Unit` is deprecated, and we can switch to `Minitest::Test` with
almost no side effects.
This commit does all of the work required to make Minitest tests run
with Gollum's existing test helpers.