tests: add mock librados API for supporting unit tests

The new librados_test_stub library can be used by unit tests
as a replacement for the librados shared library.  The library
currently provides an in-memory RADOS simulator and will
allow future unit tests to be developed using mocking.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
tests: create unittest_librbd

The existing ceph_test_librbd integration tests are now used
within unittest_librbd as unit tests run during 'make check'.
The unit tests use librados_test_stub to simulate RADOS.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>