first batch of tests

Borrowed tests from redis-py and adapted for redis-py-cluster. I removed tests for unsupported commands or added a check to make sure the call tripped an exception saying the command isn't implemented. In a few cases a call should work but doesn't or the cluster api should be able to support it to a limited extent but doesn't. In those cases I marked them as xfail so we can come back and fix them.

The pipeline tests need to be beefed up but at least we have a starting point.

Still need to set up pubsub tests, but the cluster implementation doesn't seem finished yet.