Prior to this commit, if `app/javascript/activestorage/index.js`
contained a syntax error, `JavascriptPackageTest` would still pass
because `system "yarn build"` would simply return `false` and the
compiled output would not change. This commit adds `exception: true` to
the `system` call so that an error will be raised if `yarn build` fails.
Also, since 6c96e1cd7bf3b7089c9b59b2ebeb35bfa6fb3f88, Active Storage
compiles an additional `app/assets/javascripts/activestorage.esm.js`
file. This commit adds an assertion for that file as well.
28e976b6aa