this spec needs to use the username factory

:metasploit-credential_public factory will randomly
return either a Username or BlankUsername and thus is
not appropriate for when you want tos et an explicit Username.
The :metasploit_credential_username factory should be used for this
instead

MSP-11609
use latest version of metasploit-credential

MSP-11609
metasploit-credential bump to 0.13.3

MSP-11609
Land #4197, blank username failure fix