Vendor the part of the fixtures repo we need

### What was wrong?

CI takes nearly 1minute checking out the fixtures repo, and only another 1m actually running the tests. We don't use most of the fixtures repo, and the tests should be pretty static.

### To-do

Just include the copy the folder/files we want to use for testing as a folder directly in this repo, and drop the submodule altogether. Also drop the submodule checkout step in the circleci config. CI speed should ~double.