Add a few more specs to Mdm::Service
Add a few more specs to Mdm::Service

Mdm::Service had no specs, so added some additional coverage
Tests all associations and scopes on the model

[Story #49167601]

rapid7/metasploit-model,bcook-r7/metasploit_data_models,rapid7/metasploit_data_models,rapid7/metasploit_data_models,farias-r7/metasploit_data_models,rapid7/metasploit-cache,farias-r7/metasploit_data_models,rapid7/metasploit_data_models,rapid7/metasploit-model,rapid7/metasploit-cache,bcook-r7/metasploit_data_models,farias-r7/metasploit_data_models,rapid7/metasploit-cache,rapid7/metasploit-model,bcook-r7/metasploit_data_models,rapid7/metasploit-cache