Design and implement testing strategy for examples & cookiecutter

[jackiekazil]

We need to fix tests for the examples. There were two of them in the main repo. Now they have been deactivated until we figure this out. We need to:

1. discuss/figure out testing strategy for examples and for cookiecutter & implement pinning at the same time (see Examples: To pin mesa version or not to pin? #1530)
2. Implement changes so that it is reflected in both repos (this one and the examples repo)

This ticket is to wrap up work from #963

[quaquel]

Cookiecutter got removed in #2421.

[EwoutH]

And all examples stuff is now tracked in #2364.