Describe a series of expectations for a given target desc.

TODO: find good tutorial url.

Defines a test class subclassing from either MiniTest::Spec or from the surrounding describe‘s class. The surrounding class may subclass MiniTest::Spec manually in order to easily share code:

    class MySpec < MiniTest::Spec
      # ... shared code ...
    end

    class TestStuff < MySpec
      it "does stuff" do
        # shared code available here
      end
      describe "inner stuff" do
        it "still does stuff" do
          # ...and here
        end
      end
    end