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

When a test left a timer running, jest force-stopped the worker that ran the test and printed "A worker process has failed to exit gracefully", but the run still passed. Both test scripts in package.json now run jest through script/lib/jest.js. The helper runs jest inside its own process, so the terminal output is unchanged, and it exits 1 when jest writes that warning to stderr. Jest 30 has no option that fails the run on it. The eslint.config.js comment on script/lib/ now says those files run on plain node and are never bundled.

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: {

package.json

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

script/lib/jest.js

@@ -0,0 +1,40 @@
+// 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.
+//
+// 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";
+
+// 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;
+
+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.",
+        );
+        process.exitCode = 1;
+    }
+});
+
+// Jest's own command-line program. It reads its arguments from process.argv.
+require("jest/bin/jest");