Test that yarn pack always includes the file in the "main" field

This is for compatibility with npm, which [specifies] that:

> Certain files are always included, regardless of settings:
> * package.json
> * README
> * CHANGES / CHANGELOG / HISTORY
> * LICENSE / LICENCE
> * NOTICE
> * The file in the "main" field

[specifies]: https://docs.npmjs.com/files/package.json#files

__tests__/commands/pack.js

@@ -130,7 +130,7 @@ test.concurrent('pack should include mandatory files not listed in files array i
       path.join(cwd, 'files-include-mandatory-v1.0.0.tgz'),
       path.join(cwd, 'files-include-mandatory-v1.0.0'),
     );
-    const expected = ['package.json', 'readme.md', 'license', 'changelog'];
+    const expected = ['package.json', 'index.js', 'readme.md', 'license', 'changelog'];
     expected.forEach((filename) => {
       expect(files.indexOf(filename)).toBeGreaterThanOrEqual(0);
     });

__tests__/fixtures/pack/files-include-mandatory/package.json

@@ -3,5 +3,5 @@
   "version": "1.0.0",
   "main": "index.js",
   "license": "MIT",
-  "files": ["index.js", "a.js", "b.js"]
+  "files": ["a.js", "b.js"]
 }