Finished Jest test files stay in memory on Node 24+

Jest runs every test file in its own vm context. On Node 24 and 26, once one test file runs enough tests for Jest's runner code to get optimized, the contexts of every later test file stay in memory: V8's retained maps keep them alive, and a normal full GC never frees them. Node 22 frees them.

npm install
npm run repro        # Node 24/26: "still in memory" climbs to 8
npm run repro:fixed  # --retain-maps-for-n-gc=0: "still in memory" stays at 1

counting-environment.js wraps Jest's node environment. After each test file it forces a GC and prints how many test-file globals are still in memory. Only the current file's should be.