MDL-48374 behat: new steps for detecting page loads.
MDL-48374 behat: improved page load exceptions

The following improvements have been made to the page load watching:
* Improved the exceptions when a page load expectation fails.
* Added an exception if start watching happens twice without a page load.
* Improved the page load span and xpath to make it faster and less likely
  to interfer in the future.
MDL-48425 course: management behat tests improved