Add a test that runs all of the examples

[samuelhwilliams]

A number of our example apps have had typos and small bugs in them, preventing them from actually running.

We should add a test/some tests that run all of the examples to make sure that they actually work (with their associated requirements files).

A single tox environment to run all of the examples would probably be good. In the first instance, just testing that the apps all run and a page is available on localhost:5000/admin is probably good enough. We could firm them up from there.

This test should dynamically collect all packages in the examples directory so that if any new examples are added, they are tested without having to change any config files anywhere (ie to explicitly add the new example to the test suite).