added a pytest hook to automatically skip "online" tests if client is offline

I added a new file, conftest.py, which contains a pytest hook. It iterates over all test functions with the mark *online* and checks for each of these tests if there is a working connection to the internet. If there is none, the respective test function is skipped. If this PR is merged, I recommend to use the pytest mark *online* (or anything else, I don't really care about this name) for all tests that require a connection to the internet.