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

tests/fixtures/leavesIntervalRunning.js

@@ -0,0 +1,7 @@
+// A test file that starts an interval and never clears it.
+// tests/jestRunner.test.js runs script/lib/jest.js on this file alone. The
+// name does not end in .test.js, so the normal run never picks it up.
+
+test("starts an interval and never clears it", () => {
+    setInterval(() => {}, 1000);
+});

tests/jestRunner.test.js

@@ -0,0 +1,29 @@
+// script/lib/jest.js, which package.json's test scripts run, fails a run in
+// which jest force-stopped a worker process because a test left an open
+// handle.
+
+const { spawnSync } = require("child_process");
+const path = require("path");
+
+const root = path.join(__dirname, "..");
+
+test("a test that leaves an interval running fails the run", () => {
+    // The memory limit in package.json makes jest run even this one file in a
+    // worker process. The interval keeps that worker from exiting, so jest
+    // force-stops it after the run.
+    const result = spawnSync(
+        process.execPath,
+        [
+            "script/lib/jest.js",
+            "--maxWorkers=1",
+            "--testMatch",
+            "<rootDir>/tests/fixtures/leavesIntervalRunning.js",
+        ],
+        { cwd: root, encoding: "utf8", timeout: 20000 },
+    );
+
+    expect(result.signal).toBeNull();
+    expect(result.status).toBe(1);
+    expect(result.stderr).toContain("1 passed");
+    expect(result.stderr).toContain("jest force-stopped a worker process");
+});