* Fixes support for OpenBSD (6.1+)
As reported in #219, new versions of OpenBSD ship with a different
pfioc_rule struct. This commit adjusts the offset to match the new struct.
* Fixes tests for OpenBSD 6.1+
Having the tests in a `tests` directory in root is the most common
approach. Also moved pytest's conftest.py into `tests` making the
fixture available for client and server tests.
