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

package.json

@@ -6,8 +6,8 @@
     "license": "GPL-3.0",
     "private": true,
     "scripts": {
-        "test": "node script/lib/jest.js --maxWorkers=3",
-        "test:verbose": "node script/lib/jest.js --maxWorkers=3 --verbose",
+        "test": "node script/lib/jest.js --runInBand",
+        "test:verbose": "node script/lib/jest.js --runInBand --verbose",
         "build": "node build.js",
         "lint": "eslint . && prettier --check .",
         "fmt": "prettier --write .",