Fix duplicate test case name

Extracted from #1733

It doesn't crash with shoulda-context 1.0.2 (I had to update to 1.1.6 in my PR for Rubinius), so this test was probably not even run before because of the duplication. shoulda-context now raises an exception: https://github.com/thoughtbot/shoulda-context/blob/master/lib/shoulda/context/context.rb#L392-L394

And here's the issue that added the exceptions on duplicate test name: thoughtbot/shoulda-context#26