fix(junitReporter): stamp suites with their real start time

junitReporter writes each `<testsuite timestamp>` from `suite.startedAt`,
but nothing ever set that field. Mocha's Suite does not carry it, and
`startedAt` was only assigned to individual tests, in
lib/listener/steps.js. So `toIso()` always fell through to its
`new Date()` fallback and every suite was stamped with the moment the XML
was serialized, which is after the suite (and its AfterSuite) finished.

The steps listener now stamps the suite on `event.suite.before`, mirroring
what it already does for tests.

Note this covers the in-process run. Under `run-workers` the parent only
receives `{ title }` for a test's parent suite, so the timestamp cannot
survive that boundary without changing the worker payload.

Closes #5668

Author: luantaraschi

lib/listener/steps.js

@@ -16,6 +16,14 @@ const EXCLUDED_SESSIONS = ['tryTo', 'hopeThat']
  * Register steps inside tests
  */
 export default function () {
+  // Mocha's Suite has no start timestamp of its own, and junitReporter reads
+  // `suite.startedAt` for each `<testsuite timestamp>`. Without this the
+  // reporter falls back to `new Date()` at write time, stamping every suite
+  // with the moment the XML was serialized. (#5668)
+  event.dispatcher.on(event.suite.before, suite => {
+    suite.startedAt = +new Date()
+  })
+
   event.dispatcher.on(event.test.before, test => {
     test.startedAt = +new Date()
   })

test/unit/listener/steps_suite_started_at_test.js

@@ -0,0 +1,45 @@
+import { expect } from 'chai'
+import event from '../../../lib/event.js'
+import recorder from '../../../lib/recorder.js'
+
+import stepsListener from '../../../lib/listener/steps.js'
+
+// junitReporter reads `suite.startedAt` for each `<testsuite timestamp>`.
+// Mocha never sets it, so before this listener existed the reporter fell back
+// to `new Date()` at write time. (#5668)
+describe('Steps Listener - suite.startedAt', () => {
+  beforeEach(() => {
+    recorder.reset()
+    recorder.start()
+    event.cleanDispatcher()
+    stepsListener()
+  })
+
+  afterEach(() => {
+    event.cleanDispatcher()
+    recorder.reset()
+  })
+
+  it('stamps the suite when it starts', () => {
+    const suite = { title: 'Login' }
+    const before = Date.now()
+
+    event.emit(event.suite.before, suite)
+
+    expect(suite.startedAt).to.be.a('number')
+    expect(suite.startedAt).to.be.at.least(before)
+    expect(suite.startedAt).to.be.at.most(Date.now())
+  })
+
+  it('stamps each suite independently', () => {
+    const first = { title: 'Login' }
+    const second = { title: 'Dashboard' }
+
+    event.emit(event.suite.before, first)
+    event.emit(event.suite.before, second)
+
+    expect(first.startedAt).to.be.a('number')
+    expect(second.startedAt).to.be.a('number')
+    expect(second.startedAt).to.be.at.least(first.startedAt)
+  })
+})