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

script/lib/jest.js

@@ -1,8 +1,8 @@
-// Runs jest with the arguments it is given, and fails the run when jest had to
-// force-stop the worker process that ran a test file. Jest itself only prints a
-// warning when that happens, and its run still passes. The warning means a test
-// left a timer or another open handle running. The test scripts in package.json
-// run jest through this file.
+// Runs jest with the arguments it is given, and fails the run at once when a
+// test left a timer or another open handle running. Such a handle keeps this
+// process alive after the last test, so the run would hang. Jest itself only
+// prints a warning one second after the run, and then keeps waiting. The test
+// scripts in package.json run jest through this file, in this one process.
 //
 // Jest runs inside this process and writes to the terminal directly, so its
 // output looks the same as a plain jest run. This file only checks each write
@@ -10,31 +10,23 @@
 
 "use strict";
 
-// The start of the warning that jest prints on stderr after it force-stops a
-// worker process.
-const WORKER_WARNING = "A worker process has failed to exit gracefully";
-
-let forceStopped = false;
+// The start of the warning that jest prints on stderr when the process has not
+// exited one second after the test run.
+const OPEN_HANDLE_WARNING =
+    "Jest did not exit one second after the test run has completed.";
 
 const writeStderr = process.stderr.write.bind(process.stderr);
 process.stderr.write = (chunk, ...rest) => {
-    if (String(chunk).includes(WORKER_WARNING)) {
-        forceStopped = true;
-    }
-    return writeStderr(chunk, ...rest);
-};
-
-// Runs after jest has finished, as the process exits.
-process.on("exit", () => {
-    if (forceStopped) {
-        console.error(
-            "script/lib/jest.js: failing the run, because jest force-stopped " +
-                "a worker process. A test left a timer or another open " +
-                "handle running.",
+    const written = writeStderr(chunk, ...rest);
+    if (String(chunk).includes(OPEN_HANDLE_WARNING)) {
+        writeStderr(
+            "script/lib/jest.js: failing the run, because a test left a " +
+                "timer or another open handle running.\n",
         );
-        process.exitCode = 1;
+        process.exit(1);
     }
-});
+    return written;
+};
 
 // Jest's own command-line program. It reads its arguments from process.argv.
 require("jest/bin/jest");