tests: run-tests.php leaves untracked tests/*.php artifacts after failed runs

[s2x]

Finding (from #176 follow-up, verified by subagent review)

run-tests.php extracts each .phpt --FILE-- section to tests/<basename>.php and normally unlinks it — but only when the test passes. Any failed test (fatal error, output mismatch, crash) leaves the generated .php behind.

Evidence

• run-tests.php:2618-2621 — @unlink($test_file) is inside the if ($passed) block only. Any failure skips it. (Not specific to fatal errors — any failed test leaves the file; that's stock php-src behavior for debugging.)
• tests/*.php is not git-ignored (unlike tests/*.diff|.exp|.log|.out|.sh which are), so leftovers show up as untracked files in git status.
• Observed on disk after suite runs: tests/test_query_fp64.{diff,exp,log,out,sh} artifacts from a failed expected-fail test (Aug 2026). The generated .php was manually removed.
• Relation to tests: running run-tests.php deletes legacy tracked .php test files that share a basename with .phpt #187: that issue tracks the opposite hazard of the same unlink logic (deleting tracked legacy tests/<name>.php sharing basenames). This case (leftover on failure) is distinct.

Suggested fix

• Add a cleanup note/step in AGENTS.md test_dbs/-style: after any suite run, delete untracked tests/*.php not in git ls-files
• Optionally add tests/*.php nuance to tests: running run-tests.php deletes legacy tracked .php test files that share a basename with .phpt #187's proposed fix (e.g. only unlink files that are generated, check before unlink, and clean leftovers)

Found during #176 implementation; verified by subagent review (CONFIRMED, not tracked).