Remove integration tests & coverage

The main purpose of this MR is to convert our integration tests into regular tests by leveraging the source checkout command, and not relying on any commands that require a sandbox. This should make testing of this package simpler.

While doing this, I noted that the CI was currently broken and it seems like coverage is completely busted for this package (for more details, see the commit message). This is also true for other external plugin repos like bst-external. Sorry @jennis, I hadn't noticed !1 (closed) before, but your branch also suffers from the same problem, i.e. coverage reports a 100% miss on everything.