test: remove redundant schema existence checks

[pjpjq]

Summary

• remove the redundant has_schema assertions from the migration pgTAP suite
• keep migrations/tests/database/test.sql focused on the remaining database privilege checks

The eight schemas are already covered by the Nix regression suite with stronger object/ACL snapshots:

• public, graphql_public, and realtime: schema/default ACL rows in roles.sql / roles.out
• auth: namespace owner and object snapshots in auth.sql / auth.out
• pgbouncer: namespace owner, function, and privilege snapshots in pgbouncer.sql / pgbouncer.out
• extensions: installed extension rows joined through pg_namespace in extensions_schema.sql / extensions_schema.out
• graphql: direct graphql.resolve(...) calls in pg_graphql.sql
• storage: namespace owner, table, function, and ACL snapshots in storage.sql / storage.out

Testing

• focused migration database pgTAP path in supabase/postgres:17.6.1.107: 17/17 assertions passed
• git diff --check

The complete migrations/tests/test.sql was also attempted in the same image, but the unmodified PostGIS extension setup failed before the changed database test include at pagc_normalize_address: function parse_address(character varying) does not exist.

Fixes #1561