MFC r316557:

sbuf(3): add some basic functional tests for the library

Areas not covered still [positive functionality wise] are:
- sbuf_{clear,get,set}_flags
- sbuf_new (in particular, with fixed buffers, etc).

Some basic negative testing has been added, but more will be added in the
future.

This work was in part to validate work done by cem in r288223, and ian
before that.