unit tests for pg_missing_t

All lines of code are tested. The conditions under which some methods
could corrupt the content of a pg_missing_t object have not been
investigated. Since the data members are public, the caller is
ultimately responsible for the consistency of the object and the
methods have no way to enforce it.

The semantics of is_missing have been discussed in
http://thread.gmane.org/gmane.comp.file-systems.ceph.devel/15280

Signed-off-by: Loic Dachary <loic@dachary.org>