Make tests work on PostgreSQL

https://www.pivotaltracker.com/story/show/67995534

The problem was related to how we created a temporary DataType and
DataGroup at the start of a test class, then shared it between tests.
This coupling between tests meant that one test could break another one.
It went unnoticed in SQLite because it doesn't enforce certain foreign
key constraints.

I've refactored the code so that the `_make_temp_data_group_and_type`
creates a data type/group with a long random name, and no longer deletes
it afterwards (because this was hitting database PROTECT integrity
errors).

Now instead of sharing a data type/group between tests in a test class,
each test just asks for a new pair and will always get a unique pair. This
decouples the tests from each other and makes it work in PostgreSQL.

[Fixes #67995534]
Merge pull request #120 from alphagov/make-tests-pass-on-postgresql

Make tests work on PostgreSQL