Fixed DB test cases on PyPy

They weren't working because of the intersection between some monkey
patching in the tests and the fact that ``sqlite3.Connection`` and
``Cursor`` are Python classes on PyPy, but C-extension classes on
CPython. The result was that trying to call the superclass's init
didn't work because the name ``sqlite3.Connection`` pointed to a
different class that wasn't actually the real superclass of
``GreenDBConnection``. Using ``super`` fixes this because the method
called is now always the real superclass's.

Change-Id: Ib45b16c7499883e560877788f13285d0a47cc2cd
Merge "Fixed DB test cases on PyPy"