tests: confirm that (and how) associative array values are handled in updates

[internalsystemerror]

This PR adds 2 tests:

it_adds_and_updates_a_doc()
it_updates_a_subset_of_a_doc()

This confirms that Postgres merges associative arrays but not recursively, as was possibly assumed.

php-postgres-document-store/src/PostgresDocumentStore.php

Line 366 in e9721e2

SET doc = (to_jsonb(doc) || :doc){$metadataStr}

See: event-engine/php-document-store#17 for more

(Minor note: I also had to add a test password to the postgres db to get the postgres docker container to start)