Run unit tests in CI on postgres as well as sqlite

[stevejalim]

Description

We use both postgres and sqlite, depending on the environment/situation.

At the moment, our CI runs all tests against sqlite, but it would be good if we instead (or also) them in CI against postgres.

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Success Criteria

• CI runs on postgres, either on-demand or all the time
• All tests continue to pass (which should be the case after Sanitise CMS locale lookup better + fixup existing tests that failed on postgres #16223 is merged

[janbrasna]

@stevejalim Quickly checking status quo: CI is testing in a container with make test-image which is this env config:

bedrock/docker/envfiles/test.env

Lines 1 to 5 in 95173ce

	DEBUG=False
	DEV=False
	ALLOWED_HOSTS=*
	ADMINS=["thedude@example.com"]
	WAGTAIL_ENABLE_ADMIN=True

A note in compose hints at the db service currently intended only for local dev:

bedrock/docker-compose.yml

Lines 50 to 58 in 95173ce

	# Standalone postgres database - for local runserver development.
	# TODO: add this as a dependency of app/test/release
	db:
	image: postgres:15-alpine
	environment:
	POSTGRES_PASSWORD: postgres
	ports:
	- "5432:5432"

so question is whether that would be enough to stand up for GHA test use too?

I'm thinking how to best actually start off with a useful postgres state… as it at least needs running ./bin/fill-empty-postgres-database.sh that's not part of any bootstrap scripting yet.

Whether to reuse the same container, not rebuilding from scratch, not running a separate make command with separate docker build, and exposing separate env boilerplate… and just echoing the extra config and running it again now against the db service right after it? (Sounds ugly, but may conserve some resources.)