test_runner: run top level tests in a microtask

lib/internal/test_runner/harness.js

@@ -25,6 +25,7 @@ const {
   setupTestReporters,
   shouldColorizeTestFiles,
 } = require('internal/test_runner/utils');
+const { queueMicrotask } = require('internal/process/task_queues');
 const { bigint: hrtime } = process.hrtime;
 
 const testResources = new SafeMap();
@@ -181,6 +182,7 @@ function setup(root) {
 
   root.harness = {
     __proto__: null,
+    allowTestsToRun: false,
     bootstrapComplete: false,
     coverage: FunctionPrototypeBind(collectCoverage, null, root, coverage),
     counters: {
@@ -219,7 +221,16 @@ function getGlobalRoot() {
 
 async function startSubtest(subtest) {
   await reportersSetup;
-  getGlobalRoot().harness.bootstrapComplete = true;
+
+  const root = getGlobalRoot();
+  if (!root.harness.bootstrapComplete) {
+    root.harness.bootstrapComplete = true;
+    queueMicrotask(() => {
+      root.harness.allowTestsToRun = true;
+      root.processPendingSubtests();
+    });
+  }
+
   await subtest.start();
 }
 

lib/internal/test_runner/runner.js

@@ -538,6 +538,7 @@ function run(options = kEmptyObject) {
   }
   const runFiles = () => {
     root.harness.bootstrapComplete = true;
+    root.harness.allowTestsToRun = true;
     return SafePromiseAllSettledReturnVoid(testFiles, (path) => {
       const subtest = runTestFile(path, filesWatcher, opts);
       filesWatcher?.runningSubtests.set(path, subtest);

lib/internal/test_runner/test.js

@@ -603,7 +603,7 @@ class Test extends AsyncResource {
     // it. Otherwise, return a Promise to the caller and mark the test as
     // pending for later execution.
     this.reporter.enqueue(this.nesting, this.loc, this.name);
-    if (!this.parent.hasConcurrency()) {
+    if (!this.root.harness.allowTestsToRun || !this.parent.hasConcurrency()) {
       const deferred = createDeferredPromise();
 
       deferred.test = this;

test/fixtures/test-runner/output/source_mapped_locations.snapshot

@@ -21,6 +21,9 @@ not ok 1 - fails
     *
     *
     *
+    *
+    *
+    *
   ...
 1..1
 # tests 1