#1774 test: the internal-tool leak test removes its git repositories through the shared fixture

The eight cases of tests/check-internal-tool-leak.test.js built a temporary git repository each and never removed it, the one git-building test the sweep of #1768 left out (review point on #1773). Every repository is now recorded and removed at the end through removeDirWithRetries from tests/fixtures/remove-dir.js. Test 8 of 8 locally, lint clean.

---

Summary by cubic

Stops the internal-tool leak test from leaving temporary git repositories behind after each run. Every repo the test builds is now recorded and removed through removeDirWithRetries from tests/fixtures/remove-dir, matching what the other git-building tests already do.