Add tests for looking up real and invalid postcodes
Add tests for looking up real and invalid postcodes

We should really replace the MapIt lookup with a stub that returns
expected data for these test cases, but we're leaving that for the
future.

mhl/yournextmp-popit,YoQuieroSaber/yournextrepresentative,DemocracyClub/yournextrepresentative,mysociety/yournextmp-popit,mysociety/yournextrepresentative,mhl/yournextmp-popit,datamade/yournextmp-popit,DemocracyClub/yournextrepresentative,openstate/yournextrepresentative,openstate/yournextrepresentative,neavouli/yournextrepresentative,datamade/yournextmp-popit,YoQuieroSaber/yournextrepresentative,datamade/yournextmp-popit,neavouli/yournextrepresentative,neavouli/yournextrepresentative,YoQuieroSaber/yournextrepresentative,mysociety/yournextrepresentative,mysociety/yournextmp-popit,datamade/yournextmp-popit,mysociety/yournextrepresentative,mysociety/yournextmp-popit,mhl/yournextmp-popit,neavouli/yournextrepresentative,datamade/yournextmp-popit,openstate/yournextrepresentative,openstate/yournextrepresentative,YoQuieroSaber/yournextrepresentative,openstate/yournextrepresentative,neavouli/yournextrepresentative,DemocracyClub/yournextrepresentative,YoQuieroSaber/yournextrepresentative,mysociety/yournextrepresentative,mysociety/yournextrepresentative,mysociety/yournextmp-popit,mysociety/yournextmp-popit