Add Pinia plugin for actionsCompleted in unit tests

tests/unit/testhelpers.js

 import { assert } from 'chai'
 import { cloneDeep, set as lodashSet } from 'lodash'
+import { createPinia, setActivePinia } from 'pinia'
+import { createApp, markRaw } from 'vue'
 import MockAdapter from 'axios-mock-adapter'
 import { findHandler } from 'axios-mock-adapter/src/utils'
 
@@ -211,3 +213,54 @@ export const StoreTestPlugin = store => {
    */
   store.setState = (path, value) => store._withCommit(() => lodashSet(store.state, path, value))
 }
+
+/**
+ * A Pinia store plugin that adds a `.actionsCompleted()` helper to each store and to the Pinia instance,
+ * allowing to wait for all Pinia actions to finish, successfully or not.
+ *
+ * Waiting for `store.actionsCompleted()` will only wait for the actions of this specific store.
+ * `pinia.actionsCompleted()` will wait for all actions of all stores created with this Pinia instance.
+ *
+ * @param {import('pinia').PiniaPluginContext} param0
+ */
+export function actionsCompletedPlugin ({ pinia, store }) {
+  store._promises = markRaw([])
+
+  store.actionsCompleted = markRaw(() => Promise.allSettled(store._promises))
+
+  store.$onAction(({ after, onError }) => {
+    const promise = new Promise((resolve, reject) => {
+      // ESLint thinks this after() call is a Mocha after hook and not the Pinia hook
+      // eslint-disable-next-line mocha/no-top-level-hooks
+      after(resolve)
+      onError(reject)
+    }).finally(() => {
+      const index = store._promises.indexOf(promise)
+      if (index >= 0) store._promises.splice(index, 1)
+    })
+    store._promises.push(promise)
+  })
+
+  if (!pinia.actionsCompleted) {
+    pinia.actionsCompleted = markRaw(
+      () => Promise.allSettled([...pinia._s.values()].map(store => store.actionsCompleted()))
+    )
+  }
+}
+
+/**
+ * Set up a Vue app and a Pinia instance with the specified Pinia plugins, for use only in unit tests.
+ * This should be called before calling any useStore functions.
+ * Component tests should use `createTestingPinia` from the `@pinia/testing` package instead.
+ *
+ * @param {import('pinia').PiniaPlugin[]} plugins Array of plugins to use on the Pinia instance.
+ * @returns {[import('pinia').Pinia, app: import('vue').App]} The created Pinia instance and Vue app.
+ */
+export function setUpTestPinia (plugins = []) {
+  const app = createApp({})
+  const pinia = createPinia()
+  app.use(pinia)
+  setActivePinia(pinia)
+  plugins.forEach(plugin => pinia.use(plugin))
+  return [pinia, app]
+}