* Fixes issue with #4504 and importvariable copying
ImportVariables widget was using $tw.utils.extend to copy the
variables from temporary set widgets into itself. However,
$tw.utils.extend does NOT behave like Object.assign. It not only
copies all self-owned variables over, but also all variables
in that object's prototype chain. This led to some redundant copying,
and a problem where some variables might show up more than once
(like transclusion).
Fixed now. importvariables widget does its own copying, since it
can't rely on $tw.utils.extend to do the right job, and it can't
count on Object.assign to be there.
* Added test to prevent reversion of #4504
* Slight corrections to new importvariables test
561662782e