We were sending a new request without checking the previous one had
finished, and if it hadn't finished, the test failed.
This test started to fail after upgrading to Rails 5, since we removed
the change done in commit eda47eff which set `config.allow_concurrency`
to `false` in the test environment.
While we could re-introduce that configuration option, which might have
side effects, an easier solution is to check an AJAX request has been
completed before sending a new request depending on the previous one
seems to be a more solid option.
Note this test failed with two possible errors: "undefined method
`heading' for nil:NilClass" and "stale element reference: element is not
attached to the page document". This change fixes the second error; it
might fix the first error as well, but since I couldn't reproduce it
locally, we'll only be sure when this test stops failing in travis
builds.
ca40e657fb