Fix test target coverage and add single-test runners

[seanwevans]

Motivation

• test/sql/search_path_qualification_test.sql was present in the manifest but not included in the default core test list, causing inconsistent default coverage.
• The test-triage documentation documents focused single-test targets but the Makefile lacked test-one/test-one-verbose, making local triage harder.

Description

• Added test/sql/search_path_qualification_test.sql to the CORE_TESTS list so it runs as part of the default make test flow.
• Added test-one and test-one-verbose Make targets that run a single SQL test file using pg_prove and include usage guards when TEST is unset.
• Updated .PHONY to include the new targets and placed the new targets in the Makefile after the existing test-all target.

Testing

• Ran pytest -q, which reported that no pytest tests exist in this repository (no tests ran).
• Attempted make test, which could not run the SQL test harness in this environment due to a missing PGXS include file at /usr/lib/postgresql/16/lib/pgxs/src/makefiles/pgxs.mk, so pg_prove-based tests were not executed here.

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Codex Task