Fix order-dependent test when linting audit event factories

Previously if you ran this:

```
FOSS_ONLY=1 bundle exec rspec spec/models/audit_event_spec.rb \
    spec/support/shared_examples/lint_factories_shared_examples.rb
```

The shared example file hooks into whatever example groups are loading
and attempts to lint them, which is why the audit event model has to
run.

This would fail with `NameError: uninitialized constant
Gitlab::Audit::InstanceScope` because instance-type audit events are
only implemented in EE. To avoid this error, only define the
`instance_type` trait in EE.