tests: switch to minitest 5

Adjust the test suite to support Minitest 5's syntax.

Minitest versions 4 and below do not support the newer Minitest::Test
class that arrived in version 5. For that case, use the
MiniTest::Unit::TestCase class as a fallback.
Merge pull request #62 from ktdreyer/minitest

tests: switch to minitest 5