test_runner: move coverage collection to root.postRun()

This commit moves code coverage collection from the test
harness exit handler to the postRun() function of the root
test.

This is necessary preparatory work for supporting
code coverage with --test. The reason is that --test is
implemented on top of run(), and that function calls the root
test's postRun() function, which outputs the test summary. This
happens before the harness exit handler.

PR-URL: #47651
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

lib/internal/test_runner/harness.js

@@ -1,6 +1,7 @@
 'use strict';
 const {
   ArrayPrototypeForEach,
+  FunctionPrototypeBind,
   PromiseResolve,
   SafeMap,
 } = primordials;
@@ -138,7 +139,6 @@ function setup(root) {
     createProcessEventHandler('unhandledRejection', root);
   const coverage = configureCoverage(root, globalOptions);
   const exitHandler = () => {
-    root.harness.coverage = collectCoverage(root, coverage);
     root.postRun(new ERR_TEST_FAILURE(
       'Promise resolution is still pending but the event loop has already resolved',
       kCancelledByParent));
@@ -165,7 +165,7 @@ function setup(root) {
   root.harness = {
     __proto__: null,
     bootstrapComplete: false,
-    coverage: null,
+    coverage: FunctionPrototypeBind(collectCoverage, null, root, coverage),
     counters: {
       __proto__: null,
       all: 0,

lib/internal/test_runner/test.js

@@ -644,8 +644,10 @@ class Test extends AsyncResource {
       this.reporter.diagnostic(this.nesting, kFilename, `todo ${this.root.harness.counters.todo}`);
       this.reporter.diagnostic(this.nesting, kFilename, `duration_ms ${this.#duration()}`);
 
-      if (this.harness?.coverage) {
-        this.reporter.coverage(this.nesting, kFilename, this.harness.coverage);
+      const coverage = this.harness.coverage();
+
+      if (coverage) {
+        this.reporter.coverage(this.nesting, kFilename, coverage);
       }
 
       this.reporter.push(null);