Add tests for extending ESLint

[ianschmitz]

This is a good first PR!

We should add some tests to cover #7036. They should cover both JavaScript and TypeScript support.

They can probably be added to test/fixtures.

[dscanlan]

@ianschmitz I'll take a look at this if no one else is currently looking.

[iansu]

@dscanlan it's all yours!

[kartik-budhiraja]

I would love to take this issue if you are not working on it. @dscanlan

[dscanlan]

Sure, @kartik-budhiraja . Sorry i've not got round to this..

[kushthedude]

I would love to work on it @ianschmitz

[ianschmitz]

@kartik-budhiraja you want to work on this? Giver! If not @kushthedude can take it.