Bulk assignment changes can randomly cause behat fails

The assignment module supports bulk changes to assignments. When doing so a confirmation is displayed to users. If the user cancels then nothing happens, otherwise the page reloads.

Unfortunately there are some cases where behat can try to move to the next step before the reload occurs. This is more likely to happen on faster machines, especially those with the newer W3C WebDriver.

We can apply a simple pendingJS check to prevent this from happening.