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/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);
+});