Further tests

[KillingSpark]

I need some place to keep track of the tests I want to write before I feel comfortable doing a new release

1. [x] Tests for drain_to_writer() with writers that actually do interesting things like
   1. [x] short writes
   2. [x] return E_WOULDBLOCK
2. [x] More tests for the ringbuffer. It passes a lot of random valid zstd test corpus files but I would like to test the ringbuffer specific edgecases explicitly