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

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 },