Adds a waitForMutation helper for VueX

spec/javascripts/helpers/vue_test_utils_helper.js

-/* eslint-disable import/prefer-default-export */
-
 const vNodeContainsText = (vnode, text) =>
   (vnode.text && vnode.text.includes(text)) ||
   (vnode.children && vnode.children.filter(child => vNodeContainsText(child, text)).length);
@@ -19,3 +17,19 @@ export const shallowWrapperContainsSlotText = (shallowWrapper, slotName, text) =
   Boolean(
     shallowWrapper.vm.$slots[slotName].filter(vnode => vNodeContainsText(vnode, text)).length,
   );
+
+/**
+ * Returns a promise that waits for a mutation to be fired before resolving
+ * NOTE: There's no reject action here so it will hang if it waits for a mutation that won't happen.
+ * @param {Object} store - The Vue store that contains the mutations
+ * @param {String} expectedMutationType - The Mutation to wait for
+ */
+export const waitForMutation = (store, expectedMutationType) =>
+  new Promise(resolve => {
+    const unsubscribe = store.subscribe(mutation => {
+      if (mutation.type === expectedMutationType) {
+        unsubscribe();
+        resolve();
+      }
+    });
+  });