tests: Add a buf_ring test

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.

Reviewed by: markj
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D46147