Improve within_strategy_row spec helper

Add js css class to decouple helper from DOM structure

ee/app/assets/javascripts/feature_flags/components/form.vue

@@ -305,7 +305,7 @@ export default {
             </div>
           </div>
         </div>
-        <template v-if="filteredStrategies.length > 0">
+        <div v-if="filteredStrategies.length > 0" data-testid="feature-flag-strategies">
           <strategy
             v-for="(strategy, index) in filteredStrategies"
             :key="strategy.id"
@@ -316,7 +316,7 @@ export default {
             @change="onFormStrategyChange($event, index)"
             @delete="deleteStrategy(strategy)"
           />
-        </template>
+        </div>
         <div v-else class="flex justify-content-center border-top py-4 w-100">
           <span>{{ $options.translations.noStrategiesText }}</span>
         </div>

ee/spec/support/helpers/feature_flag_helpers.rb

@@ -33,7 +33,7 @@ module FeatureFlagHelpers
   end
 
   def within_strategy_row(index)
-    within ".feature-flags-form > fieldset > div:nth-child(#{index + 3})" do
+    within ".feature-flags-form > fieldset > div[data-testid='feature-flag-strategies'] > div:nth-child(#{index})" do
       yield
     end
   end