Flush out the operator crd api tests
Summary
Part of https://gitlab.com/charts/gitlab/issues/945
Our operator currently has an api test for the GitLab type that just ensures that the most basic instance of the object can be created, updated, and destroyed. https://gitlab.com/charts/components/gitlab-operator/blob/master/pkg/apis/gitlab/v1beta1/gitlab_types_test.go
We need to flush out these tests to expand beyond the default, to at least exercise the required fields.
This is less to test kubernetes' handling of required fields, and more to ensure our expectations for the crd are in tests. The crd is largely auto generated by kubebuilder, so these tests will ensure we don't accidentally introduce a change that causes an unexpected results.