Refactor test_password_hashed to the backend testers

The test_password_hashed method was defined in
test_backend.IdentityTests. This is a bad place for it because not
all backends do password hashing (for example, the LDAP backend
doesn't), and it was also implemented in a way where it called a
private method of the backend, which the SQL backend implemented
differently (it requires an SQLAlchemy session).

The test_password_hashed method is moved into the KVS backend
test class, since the current implementation is specific to the KVS
backend.

Change-Id: I0a2390412de6304b7436e6723cb2d411dd53421d
Merge "Refactor test_password_hashed to the backend testers"