fix: Await for persistVirtualDocuments result before finishing request

In previous implementation we did not await for the
`persistVirtualDocuments()` result before completing the request

This was done because the persistance was not mandatory for the
request's result and doing it in parallel would be a good way to
optimize timings

But in some recent experiments we found a scenario where we would
benefit from the `await` in order to some external code until the data
is actually persisted

Related PR: #1486

packages/cozy-client/src/CozyClient.js

@@ -1093,7 +1093,7 @@ client.query(Q('io.cozy.bills'))`)
   async requestQuery(definition) {
     const mainResponse = await this.chain.request(definition)
 
-    this.persistVirtualDocuments(definition, mainResponse.data)
+    await this.persistVirtualDocuments(definition, mainResponse.data)
 
     if (!definition.includes) {
       return mainResponse

packages/cozy-client/src/associations.spec.js

@@ -7,7 +7,8 @@ import { SCHEMA, TODO_1, TODO_2 } from './__tests__/fixtures'
 
 describe('Associations', () => {
   const requestHandler = jest.fn()
-  const link = new CozyLink(requestHandler)
+  const persistHandler = jest.fn()
+  const link = new CozyLink(requestHandler, persistHandler)
   const client = new CozyClient({ links: [link], schema: SCHEMA })
 
   const getTodo = id =>