This is a reboot of MDL-41742.

When using goutte driver, the DOM will never change after the initial loading and so it is pointless to wait for changes in spin().

Against master, time to run behat with tags '~@javascript&&@mod_forum'...
before: 3m36.416s
after: 1m10.482s

Unlike my previous patch, this doesn't speed up or otherwise affect the polling frequency in javascript mode, so there is not a negative performance impact there.