test_runner: avoid overwriting root start time

lib/internal/test_runner/test.js

@@ -608,7 +608,7 @@ class Test extends AsyncResource {
     if (this.parent !== null) {
       this.parent.activeSubtests++;
     }
-    this.startTime = hrtime();
+    this.startTime ??= hrtime();
 
     if (this[kShouldAbort]()) {
       this.postRun();

test/fixtures/test-runner/root-duration.mjs

@@ -0,0 +1,7 @@
+import { test, after } from 'node:test';
+
+after(() => {});
+
+test('a test with some delay', (t, done) => {
+  setTimeout(done, 50);
+});

test/parallel/test-runner-root-duration.js

@@ -0,0 +1,25 @@
+'use strict';
+const { spawnPromisified } = require('../common');
+const fixtures = require('../common/fixtures');
+const { strictEqual } = require('node:assert');
+const { test } = require('node:test');
+
+test('root duration is longer than test duration', async () => {
+  const {
+    code,
+    stderr,
+    stdout,
+  } = await spawnPromisified(process.execPath, [
+    fixtures.path('test-runner/root-duration.mjs'),
+  ]);
+
+  strictEqual(code, 0);
+  strictEqual(stderr, '');
+  const durations = [...stdout.matchAll(/duration_ms:? ([.\d]+)/g)];
+  strictEqual(durations.length, 2);
+  const testDuration = Number.parseFloat(durations[0][1]);
+  const rootDuration = Number.parseFloat(durations[1][1]);
+  strictEqual(Number.isNaN(testDuration), false);
+  strictEqual(Number.isNaN(rootDuration), false);
+  strictEqual(rootDuration >= testDuration, true);
+});