test: improve coverage for load hooks

Refs: nodejs#43363 (comment)

test/es-module/test-esm-loader-thenable.mjs

@@ -0,0 +1,65 @@
+import { mustCall } from '../common/index.mjs';
+import { fileURL, path } from '../common/fixtures.mjs';
+import { match, ok, notStrictEqual, strictEqual } from 'assert';
+import { spawn } from 'child_process';
+import { execPath } from 'process';
+
+{
+  const child = spawn(execPath, [
+    '--experimental-loader',
+    fileURL('es-module-loaders', 'thenable-load-hook.mjs').href,
+    path('es-modules', 'test-esm-ok.mjs'),
+  ]);
+
+  let stderr = '';
+  child.stderr.setEncoding('utf8');
+  child.stderr.on('data', (data) => {
+    stderr += data;
+  });
+  child.on('close', mustCall((code, _signal) => {
+    strictEqual(code, 0);
+    ok(!stderr.includes('must not call'));
+  }));
+}
+
+{
+  const child = spawn(execPath, [
+    '--experimental-loader',
+    fileURL('es-module-loaders', 'thenable-load-hook-rejected.mjs').href,
+    path('es-modules', 'test-esm-ok.mjs'),
+  ]);
+
+  let stderr = '';
+  child.stderr.setEncoding('utf8');
+  child.stderr.on('data', (data) => {
+    stderr += data;
+  });
+  child.on('close', mustCall((code, _signal) => {
+    notStrictEqual(code, 0);
+
+    match(stderr, /\sError: must crash the process\n/);
+
+    ok(!stderr.includes('must not call'));
+  }));
+}
+
+{
+  const child = spawn(execPath, [
+    '--experimental-loader',
+    fileURL('es-module-loaders', 'thenable-load-hook-rejected-no-arguments.mjs').href,
+    path('es-modules', 'test-esm-ok.mjs'),
+  ]);
+
+  let stderr = '';
+  child.stderr.setEncoding('utf8');
+  child.stderr.on('data', (data) => {
+    stderr += data;
+  });
+  child.on('close', mustCall((code, _signal) => {
+    notStrictEqual(code, 0);
+
+    match(stderr, /\sundefined\n/);
+
+    ok(!stderr.includes('must not call'));
+  }));
+}

test/fixtures/es-module-loaders/thenable-load-hook-rejected-no-arguments.mjs

@@ -0,0 +1,10 @@
+export function load () {
+  let thenAlreadyAccessed = false;
+  return {
+    get then() {
+      if (thenAlreadyAccessed) throw new Error('must not call');
+      thenAlreadyAccessed = true;
+      return (_, reject) => reject();
+    }
+  };
+}

test/fixtures/es-module-loaders/thenable-load-hook-rejected.mjs

@@ -0,0 +1,10 @@
+export function load () {
+  let thenAlreadyAccessed = false;
+  return {
+    get then() {
+      if (thenAlreadyAccessed) throw new Error('must not call');
+      thenAlreadyAccessed = true;
+      return (_, reject) => reject(new Error('must crash the process'));
+    }
+  };
+}

test/fixtures/es-module-loaders/thenable-load-hook.mjs

@@ -0,0 +1,10 @@
+export function load(url, context, next) {
+  let thenAlreadyAccessed = false;
+  return {
+    get then() {
+      if (thenAlreadyAccessed) throw new Error();
+      thenAlreadyAccessed = true;
+      return (resolve) => resolve(next(url, context));
+    }
+  };
+}