testing: Add a system-provided postgres for testing

[cdecker]

• testing: Add a system-postgres mode for faster testing

Postgres can be slow to spin up and down clusters for each test. This
patch allows us to just test against an externally managed postgres
instance. This is particularly interesting in CI, where we can run a
postgres service in a separate container.

The provider still uses nonce-based multitenancy. It tries to clean
up, but it can fail if a test gets interrupted or teardown fails.

Checklist

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• The changelog has been updated in the relevant commit(s) according to the guidelines.
• [-] Tests have been added or modified to reflect the changes.
• [-] Documentation has been reviewed and updated as needed.
• Related issues have been listed and linked, including any that this PR closes.

Changelog-None