Mock results from the Google geocoder API

These tests would intermittently fail due to network problems, or
results changing as Google include more places.  We should only be
testing our code, not the API, so replace the data returned by this
API with saved serialized data.
Make hansard's test_source tests work without a network

These tests actually tried to fetch files from www.mysociety.org,
which wasn't testing anything useful.  This commit changes the
tests so that they don't require the network, but still test
the same parts of our code.
Reformat lines that are too long in the geocoder tests
Remove unused imports from test_source.py