[devtools] qunit: use new async testing APIs
http://qunitjs.com/cookbook/#asynchronous-callbacks
QUnit keeps track of all the assert.async() objects created inside the
test functions and expects all done() functions to be called. Failure to
do so will result in the test being failed.
Unlike .start and .stop which were internal APIs, assert.async() is
stricter and fails tests if assert methods are used *after* all done()
functions are called (see "test callback execution order").
Related to #5533333.
Templates
---------
*Templates* are specific views that do not depend on a result set. The basic
class `Template` (`cubicweb.common.view`) is derived from the class `View`.
To build a HTML page, a *main template* is used. In general, the template of
identifier `main` is the one to use (it is not used in case an error is raised or for
the login form for example). This template uses other templates in addition
to the views which depends on the content to generate the HTML page to return.
A *template* is responsible for:
1. executing RQL query of data to render if necessary
2. identifying the view to use to render data if it is not specified
3. composing the HTML page to return
You will find out more about templates in :ref:`templates`.