tests: switch to minitest

Ruby 1.9+ uses Minitest as the backend for Test::Unit. As of Minitest 5,
the shim no longer supports Test::Unit::TestCase.

Adjust the posix-spawn test suite to support Minitest 5's syntax.
tests: refactor requires into test_helper

Move the duplicated "requires" lines into a test_helper script.
tests: add Minitest 4 backwards-compatibility

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.