Add unittests for clone_unsharable.

Checks three assertions:

1. a bufferlist shared with buffer::list::share yields shared
raw memory

2. a bufferlist copied with buffer::list::list(const list&) yields
shared raw memory

3. an unsharable bufferlist (bufferlist with unsharable segments)
copied with buffer::list::list(const list&) yields unshared raw
memory

Signed-off-by: Matt Benjamin <matt@cohortfs.com>