Add tests for management commands

[DylannCordel]

Currently, there are no tests for all management commands :

• pybb/management/commands/dump_topics.py
• pybb/management/commands/migrate_profile.py
• pybb/management/commands/pybb_delete_invalid_topics.py
• pybb/management/commands/pybb_update_counters.py
• pybb/management/commands/supermoderator.py

Writing test is cool and is a good way to begin to participate to a project if some people are interested by coding this 🐱