Fix random spec failure
Fix random spec failure

The spec needs to guarantee the (not namespaced) resource is actually
defined so the `remove_const` on the controller will succeed.

Fixes

```
rspec \
  ./spec/unit/namespace/register_resource_spec.rb[1:5:4:2:1] \
  ./spec/unit/namespace_spec.rb[1:2:1:1] \
  --seed 22761
```

deivid-rodriguez/activeadmin,vraravam/activeadmin,gogovan/activeadmin,mrjman/activeadmin,vraravam/activeadmin,waymondo/active_admin,mauriciopasquier/active_admin,varyonic/activeadmin,deivid-rodriguez/activeadmin,mrjman/activeadmin,cmunozgar/activeadmin,varyonic/activeadmin,vraravam/activeadmin,waymondo/active_admin,mrjman/activeadmin,javierjulio/activeadmin,cmunozgar/activeadmin,maysam/activeadmin,vytenis-s/activeadmin,varyonic/activeadmin,maysam/activeadmin,mauriciopasquier/active_admin,SoftSwiss/active_admin,cmunozgar/activeadmin,wspurgin/activeadmin,siutin/activeadmin,activeadmin/activeadmin,wspurgin/activeadmin,deivid-rodriguez/activeadmin,javierjulio/activeadmin,siutin/activeadmin,vytenis-s/activeadmin,javierjulio/activeadmin,siutin/activeadmin,SoftSwiss/active_admin,gogovan/activeadmin,vytenis-s/activeadmin,SoftSwiss/active_admin,mauriciopasquier/active_admin,activeadmin/activeadmin,waymondo/active_admin,activeadmin/activeadmin,wspurgin/activeadmin,maysam/activeadmin,gogovan/activeadmin