tests: Add a buf_ring test

[andrew]

Summary

Add a simple test for sys/buf_ring.h. This enqueues and dequeues items
in the buf_ring to ensure the simple operations are correct.

There are no checks for multiple threads operating on buf_ring_enqueue
or buf_ring_dequeue_mc as they can be probabilistic and depend on the
architecture memory model.

[markj]

tests/sys/sys/buf_ring_test.c

20

I understand why this is convenient (otherwise you need to buildworld to pick up the in-tree buf_ring.h header), but other tests in this directory don't behave like this. IMO it'd be better to be consistent.

28

Should we define these in buf_ring.h?

[andrew]

tests/sys/sys/buf_ring_test.c

28

We could. I put them here to allow for a non-trivial implementation.

[andrew]

Use #include <sys/buf_ring.h>