minitest, test::unit, and rails
There is a Test::Unit "compatibility" module that comes with Minitest, so that you can (presumably) use your existing Test::Unit tests as-is. This is probably the Test::Unit module you are seeing.
As of rails 3.2.3, generator-created tests include rails/test_help which includes test/unit.
The test "something" do syntax is a rails extension. It's defined in ActiveSupport::Testing::Declarative, which is require'd by rails/test_help.
This is perhaps a bit of a tangential response, but as to your rspec comment... You might want to take a look at minitest/spec which provides spec flavor syntax in stdlib in 1.9.x.