test: replace jest workers past 200 MB so they exit in time (closes #766)

On a starved CPU, jest force-stopped worker processes that could not exit
within the 500 ms it allows, and script/lib/jest.js failed the run although
every test passed. No test leaks a handle; exit time grows with what a
worker holds, 380 to 970 MB at the end of the run.

workerIdleMemoryLimit in package.json replaces a worker whose heap is over
200 MB after a test file with a fresh one.

tests/jestRunner.test.js runs script/lib/jest.js on a fixture that leaves
an interval running and expects the force-stop to fail the run.

Model: opus-5-5

eslint.config.js

@@ -135,9 +135,14 @@ module.exports = [
         },
     },
 
-    // Unit tests, and the helpers they require: jest on node.
+    // Unit tests, the helpers they require, and the test files they hand to
+    // jest: jest on node.
     {
-        files: ["tests/**/*.test.js", "tests/support/**/*.js"],
+        files: [
+            "tests/**/*.test.js",
+            "tests/support/**/*.js",
+            "tests/fixtures/**/*.js",
+        ],
         languageOptions: {
             ...commonjs,
             globals: { ...globals.node, ...globals.jest },