test: run jest in one process, fail on an open handle (closes #766)

Jest force-stopped worker processes that could not exit within 500 ms on
a starved CPU, failing runs where every test passed. The test scripts now
pass --runInBand. script/lib/jest.js fails the run at once when jest
warns that it did not exit one second after the run, instead of
hanging. The Dockerfile test stage drops its 90-second timeout.

tests/jestRunner.test.js runs the runner on a fixture that leaves an
interval running and expects exit status 1.

Model: opus-5-5

tests/jestRunner.test.js

@@ -0,0 +1,29 @@
+// script/lib/jest.js, which package.json's test scripts run, fails a run in
+// which a test left an open handle, instead of hanging on it.
+
+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 run ends about one second after its last test. Without the check in
+    // script/lib/jest.js it would never end, and the timeout stops it.
+    const result = spawnSync(
+        process.execPath,
+        [
+            "script/lib/jest.js",
+            "--runInBand",
+            "--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(
+        "a test left a timer or another open handle running",
+    );
+});