Add a DummyApp sprockets configuration manifest

[kennyadsl]

Description Sprockets 4 introduced a mandatory configuration manifest at a specific location. We don't have the ability to set it at that specific location in the DummyApp we use for our specs.

I've opened https://github.com/rails/sprockets-rails/pull/446 on sprockets-rails which should fix this issue. I'll leave this PR open (working with that branch) waiting to know if that PR will be accepted.

Closes #3374, closes #3376

After merging this we need to revert #3378,.

Checklist:

• [ ] I have followed Pull Request guidelines
• [ ] I have added a detailed description into each commit message
• [ ] I have updated Guides and README accordingly to this change (if needed)
• [ ] I have added tests to cover this change (if needed)