Adds cluster_for_group factory for convienence

Also means we don't have to resort to an update statement to set parent
for child groups who also have clusters.

This is much shorter than

```
create(:cluster, :provided_by_gcp, :group, groups: [group])
```

spec/factories/clusters/clusters.rb

@@ -5,6 +5,8 @@ FactoryBot.define do
     cluster_type :project_type
     managed true
 
+    factory :cluster_for_group, traits: [:provided_by_gcp, :group]
+
     trait :instance do
       cluster_type { Clusters::Cluster.cluster_types[:instance_type] }
     end

spec/models/concerns/deployment_platform_spec.rb

@@ -46,12 +46,11 @@ describe DeploymentPlatform do
       end
 
       context 'when child group has configured kubernetes cluster', :nested_groups do
-        let!(:child_group1_cluster) { create(:cluster, :provided_by_gcp, :group) }
-        let(:child_group1) { child_group1_cluster.group }
+        let(:child_group1) { create(:group, parent: group) }
+        let!(:child_group1_cluster) { create(:cluster_for_group, groups: [child_group1]) }
 
         before do
           project.update!(group: child_group1)
-          child_group1.update!(parent: group)
         end
 
         it 'returns the Kubernetes platform for the child group' do
@@ -59,11 +58,10 @@ describe DeploymentPlatform do
         end
 
         context 'deeply nested group' do
-          let!(:child_group2_cluster) { create(:cluster, :provided_by_gcp, :group) }
-          let(:child_group2) { child_group2_cluster.group }
+          let(:child_group2) { create(:group, parent: child_group1) }
+          let!(:child_group2_cluster) { create(:cluster_for_group, groups: [child_group2]) }
 
           before do
-            child_group2.update!(parent: child_group1)
             project.update!(group: child_group2)
           end