Swap unconditional expose test with schema check

Though we still need to verify that rollout_status is present or not as
schema check can't do that for us

ee/spec/fixtures/api/schemas/clusters/environment.json

@@ -19,7 +19,10 @@
       ]
     },
     "rollout_status": {
-      "$ref": "../rollout_status.json"
+      "oneOf": [
+        { "type": "null" },
+        { "$ref": "../rollout_status.json" }
+      ]
     },
     "updated_at": { "type": "date" }
   },

ee/spec/serializers/clusters/environment_entity_spec.rb

@@ -22,28 +22,16 @@ describe Clusters::EnvironmentEntity do
 
     subject { described_class.new(environment, request: request).as_json }
 
-    it 'exposes project' do
-      expect(subject).to include(:project)
-    end
-
-    it 'exposes last_deployment' do
-      expect(subject).to include(:last_deployment)
-    end
-
-    it 'exposes environment_path' do
-      expect(subject).to include(:environment_path)
-    end
-
-    it 'exposes updated_at' do
-      expect(subject).to include(:updated_at)
-    end
-
     context 'deploy board available' do
       before do
         allow(group).to receive(:feature_available?).and_call_original
         allow(group).to receive(:feature_available?).with(:cluster_deployments).and_return(true)
       end
 
+      it 'matches expected schema' do
+        expect(subject.with_indifferent_access).to match_schema('clusters/environment', dir: 'ee')
+      end
+
       it 'exposes rollout_status' do
         expect(subject).to include(:rollout_status)
       end
@@ -54,6 +42,10 @@ describe Clusters::EnvironmentEntity do
         allow(group).to receive(:feature_available?).with(:cluster_deployments).and_return(false)
       end
 
+      it 'matches expected schema' do
+        expect(subject.with_indifferent_access).to match_schema('clusters/environment', dir: 'ee')
+      end
+
       it 'does not expose rollout_status' do
         expect(subject).not_to include(:rollout_status)
       end