Remove redundant tests in `migrations/tests/database/exists.sql`

[steve-chavez]

Problem

We currently have these tests:

postgres/migrations/tests/database/exists.sql

Lines 2 to 8 in 61ff753

	SELECT has_schema('public');
	SELECT has_schema('auth');
	SELECT has_schema('extensions');
	SELECT has_schema('graphql');
	SELECT has_schema('graphql_public');
	SELECT has_schema('realtime');
	SELECT has_schema('storage');

As one example, has_schema('storage'); is redundant with

postgres/nix/tests/expected/storage.out

Lines 1 to 14 in 61ff753

	-- storage schema owner
	select
	n.nspname as schema_name,
	r.rolname as owner
	from
	pg_namespace n
	join
	pg_roles r on n.nspowner = r.oid
	where
	n.nspname = 'storage';
	schema_name | owner
	-------------+----------------
	storage | supabase_admin
	(1 row)

Which provides more information.

Editing the exists.sql file makes up for more work on PRs (example) while not providing any more test coverage.

Additionally, it's easier to locate all storage concerns in a single file, the same for the other schemas.

Solution

Remove migrations/tests/database/exists.sql while ensuring the test coverage is preserved.