Fix flaky test in manifest_route_update_spec

Use route_mappings_dataset to defer query and avoid race condition.

Author: joyvuu-dave

spec/unit/actions/manifest_route_update_spec.rb

@@ -66,7 +66,7 @@ module VCAP::CloudController
             it 'updates (or recreates) the route mapping with the new protocol' do
               ManifestRouteUpdate.update(app.guid, message, user_audit_info)
 
-              route_mappings = app.reload.route_mappings
+              route_mappings = app.reload.route_mappings_dataset
 
               expect(route_mappings.count).to eq(2)
               mapped_route = route_mappings.find { |rm| rm.route == route }