Test coverage

There's un-tested code:

1. This if block is never touched during tests: https://git.geomar.de/python/xorca_brokenline/blob/master/xorca_brokenline/init.py#L126

2. Reducing the grid never drops anything: https://git.geomar.de/python/xorca_brokenline/blob/master/xorca_brokenline/init.py#L35

See coverage report at the end of the job (e.g. https://git.geomar.de/python/xorca_brokenline/-/jobs/5459) for details.