src/test: fix to avoid fail notification when testing manifest refcount

src/test: fix to avoid fail notification when testing manifest refcount

Due to false-positive design on manifest snap refcounting,
a message to decrement the refcount can be missing.
This commit checks whether the manifest object's state is correct
when such mismatch happens to prevent aborting unit test.

Signed-off-by: Myoungwon Oh <myoungwon.oh@samsung.com>