Never drop the database when granting privileges

The default behavior of seed_fu is to load the fixtures using the RAILS_ENV
environment.  In CI, since we set RAILS_ENV=test, nothing is ever
loaded. Instead of `rake db:seed_fu`, use `rake gitlab:setup`, which sets up
MySQL properly with limits.

Closes #41517

We have run into permission issues with MySQL triggers in #36633 that
would have been caught earlier either if our migration tests or GitLab QA
tests had been testing against non-superuser users. This change creates
a non-superuser that has access to the GitLab test database and uses that.

Closes #39932