Optimise the behat feature for the Feel > Blocks settings (moodle-an-…

…hochschulen#765)

Navigation steps have been optimised a bit, creation of blocks
now uses generators, and @javascript has been removed from tests
that do not require it.

Only tests using the off-canvas block region need Javascript since it
opens the overlay. Otherwise, we are just checking whether block regions
exist or not under various circumstances, which can be done much quicker
without Javascript.