Instead of using hardcoded -1 for errors, include -1 in the enum auth_action
type; apply its use.
In the mm_auth test, the string output changes from '(internal error)' to
'AUTH_ERROR', since now the proper enum value is used in auth_action_names[].
Add basic MM Authentication test setup, with fake DB access and RAND_bytes().
So far implement simple tests for IO error during DB access and missing auth
entry.
To print the auth action during tests, add struct auth_action_names and
auth_action_str() inline function in auth.[hc].