fix tests

remove old "rmtrees" from before fixture refactor

Seems I didn't notice the old tests were trying to "clean up" themselves, rather than relying on pytest. That now causes them to delete the pytest directories.

fix bug in coverage:

This is also an attempt to fix the suddent test failures with the very strange error:

CoverageWarning: Couldn't parse 
'/tmp/pytest-of-root/pytest-0/events0/ctapipe.core.expression_engine': 
No source for code: '/tmp/pytest-of-root/pytest-0/events0/ctapipe.core.expression_engine'. 
(couldnt-parse)

Edit: there were missing __init__.py files in the tests directories that are required by pytest-cov.