test_runner: print info when test restarts

lib/internal/test_runner/reporter/spec.js

@@ -5,6 +5,8 @@ const {
   ArrayPrototypePush,
   ArrayPrototypeShift,
   ArrayPrototypeUnshift,
+  Date,
+  DatePrototypeToLocaleString,
 } = primordials;
 const assert = require('assert');
 const Transform = require('internal/streams/transform');
@@ -106,6 +108,9 @@ class SpecReporter extends Transform {
         if (data.file === undefined) {
           return this.#formatFailedTestResults();
         }
+        break;
+      case 'test:watch:restarted':
+        return `\nRestarted at ${DatePrototypeToLocaleString(new Date())}\n`;
     }
   }
   _transform({ type, data }, encoding, callback) {

test/test-runner/test-output-spec-reporter-watch-restart.mjs

@@ -0,0 +1,18 @@
+import '../common/index.mjs';
+import { spec as SpecReporter } from 'node:test/reporters';
+import assert from 'node:assert';
+
+const reporter = new SpecReporter();
+let output = '';
+
+reporter.on('data', (chunk) => {
+  output += chunk;
+});
+
+reporter.write({
+  type: 'test:watch:restarted',
+});
+
+reporter.end();
+
+assert.match(output, /Restarted at .+/);