Add a test case for resolve.fallback

[dmattia]

What kind of change does this PR introduce?

This is an additional test for showing the resolve.fallback functionality, ensuring that it does not override packages that do already exist.

Did you add tests for your changes?

my changes are just a test

Does this PR introduce a breaking change?

nah

What needs to be documented once your changes are merged?

nothing

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

[webpack-bot]

Thank you for your pull request! The most important CI builds succeeded, we’ll review the pull request soon.

[sokra]

Please make sure to add the email of your commit (david@transcend.io) to your github account and sign our CLA.

[dmattia]

@sokra Awesome, thank you. I have added that email and signed the CLA

[webpack-bot]

Hi @dmattia.

Just a little hint from a friendly bot about the best practice when submitting pull requests:

Don't submit pull request from your own master branch. It's recommended to create a feature branch for the PR.

You don't have to change it for this PR, just make sure to follow this hint the next time you submit a PR.

[sokra]

Thanks