Add docs about waiting for requests

(cherry picked from commit 569d825092654fc74ccb12f2dcd3dc86c00233be)

doc/development/testing_guide/frontend_testing.md

@@ -321,8 +321,7 @@ it('waits for an Ajax call', done => {
 });
 ```
 
-If you are not able to register handlers to the `Promise`—for example because it is executed in a synchronous Vue life
-cycle hook—you can flush all pending `Promise`s:
+If you are not able to register handlers to the `Promise`, for example because it is executed in a synchronous Vue life cycle hook, please take a look at the [waitFor](#wait-until-axios-requests-finish) helpers or you can flush all pending `Promise`s:
 
 **in Jest:**
 
@@ -1088,6 +1087,16 @@ afterEach(() => {
 });
 ```
 
+### Wait until axios requests finish
+
+The axios utils mock module located in `spec/frontend/mocks/ce/lib/utils/axios_utils.js` contains two helper methods for Jest tests that spawn HTTP requests.
+These are very useful if you don't have a handle to the request's Promise, for example when a Vue component does a request as part of its life cycle.
+
+- `waitFor(url, callback)`: Runs `callback` after a request to `url` finishes (either successfully or unsuccessfully).
+- `waitForAll(callback)`: Runs `callback` once all pending requests have finished. If no requests are pending, runs `callback` on the next tick.
+
+Both functions run `callback` on the next tick after the requests finish (using `setImmediate()`), to allow any `.then()` or `.catch()` handlers to run.
+
 ## Testing with older browsers
 
 Some regressions only affect a specific browser version. We can install and test in particular browsers with either Firefox or Browserstack using the following steps: