Fix tests by introducing DSL separating DDL, relations, and seeding

[flash-gordon]

Before that, emerging dependencies between spec parts were hard to navigate: when you call container, it materializes the container, meaning you won't be able to add new tables/relations to the container. It's a mystery to me how we've managed to continue without proper separation of the stages so far. Finally, I've hit it pretty hard when fixing failing specs.

What this introduces are three DSL methods accepting blocks:

• setup_tables
• setup_relations
• seed
  No setup_relations blocks are yielded before all setup_tables are run. seed blocks follow setup_relations.

These methods have an additional optional parameter for sequencing blocks within one stage:

setup_tables(:base) {}  # called first
setup_tables(dep: :base) {} # depends on base, called second
setup_tables {} # no dependency provided, will be called last

Also, in a given spec, you can use metadata to prevent seeding or declaring relations from a shared context:

include 'users and tasks'

describe 'a block without seeding and relations', relations: false, seeds: false do
   ....
end

describe 'a block without seeding', seeds: false do
  ....
end