test: make test fails when jest force-stops a worker (closes #711)

When a test leaves a timer running, jest force-stops the worker that ran
the test, prints a warning, and the run still passes. Jest has no option
that fails the run instead. Both test scripts in package.json now run
jest through script/lib/jest.js. Jest runs inside that file's process,
so its terminal output stays the same. The file exits 1 when jest
printed that warning.

Model: opus-5-5

eslint.config.js

@@ -153,8 +153,7 @@ module.exports = [
         },
     },
 
-    // The helpers the script/ entrypoints call: plain node programs too, run
-    // from a shell script rather than from yarn, and never bundled.
+    // The files in script/lib/ run on plain node too, and are never bundled.
     {
         files: ["script/lib/**/*.js"],
         languageOptions: {

script/lib/jest.js

@@ -1,33 +1,40 @@
 // Runs jest with the arguments it is given, and fails the run when jest had to
-// force-stop a worker. 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.
+// 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.
+//
+// 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
+// to stderr for the warning below.
 
 "use strict";
 
-const { spawn } = require("child_process");
-
 // The start of the warning that jest prints on stderr after it force-stops a
-// worker.
+// worker process.
 const WORKER_WARNING = "A worker process has failed to exit gracefully";
 
-const jest = spawn("jest", process.argv.slice(2), {
-    stdio: ["inherit", "inherit", "pipe"],
-});
+let forceStopped = false;
 
-let stderr = "";
-jest.stderr.on("data", (chunk) => {
-    process.stderr.write(chunk);
-    stderr += chunk;
-});
+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);
+};
 
-jest.on("close", (code) => {
-    if (stderr.includes(WORKER_WARNING)) {
+// 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. A test left a timer or another open handle running.",
+                "a worker process. A test left a timer or another open " +
+                "handle running.",
         );
-        process.exit(1);
+        process.exitCode = 1;
     }
-    process.exit(code ?? 1);
 });
+
+// Jest's own command-line program. It reads its arguments from process.argv.
+require("jest/bin/jest");