test: add more module loader test coverage

Verify that a package.json without a .main property loads index.js.

PR-URL: #9196
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/fixtures/packages/index/index.js

@@ -0,0 +1 @@
+exports.ok = 'ok';

test/fixtures/packages/index/package.json

@@ -0,0 +1 @@
+{}

test/sequential/test-module-loading.js

@@ -69,6 +69,8 @@ assert.equal(threeFolder, threeIndex);
 assert.notEqual(threeFolder, three);
 
 console.error('test package.json require() loading');
+assert.equal(require('../fixtures/packages/index').ok, 'ok',
+             'Failed loading package');
 assert.equal(require('../fixtures/packages/main').ok, 'ok',
              'Failed loading package');
 assert.equal(require('../fixtures/packages/main-index').ok, 'ok',
@@ -208,6 +210,7 @@ assert.deepStrictEqual(children, {
   },
   'fixtures/nested-index/three.js': {},
   'fixtures/nested-index/three/index.js': {},
+  'fixtures/packages/index/index.js': {},
   'fixtures/packages/main/package-main-module.js': {},
   'fixtures/packages/main-index/package-main-module/index.js': {},
   'fixtures/cycles/root.js': {