Fix some of the ordering issues in postgres.

Postgres is far more pedantic about constraints, and tweaking the
fixture order helps.
DRY out fixture manager.

Make common code shared across the various load/truncate/drop
operations.
Tweak fixtures and expectations to make tests pass.

Now that articles_tags has a FK, it always has to accompany the tags
table, which it wasn't in the OrmCacheShellTest. The ModelTaskTest
expectations also had to change becuase of additional fixtures.
Insert data in a different transaction than schema.

Postgres will puke if you try to alter schema and update rows in the
same transaction. While this makes the tests a bit slower, it is
necessary to have foreign key fixtures work with postgres.