Refactor tests as bilby_pipe.tests subpackage, or include tests in source distribution (tarball)

It would be great to be able to execute integration-level tests for bilby_pipe on an installed version of the project. This would be best enabled by refactoring the tests/ directory as a bilby_pipe.tests subpackage of the Python library.

A backup solution would be to ensure that the tests/ path is included in source distributions (tarballs) so that the tests can be executed alongside package builds.