HAS_EXTENDED_CREATE_ELEMENT_SYNTAX may be true only in IE8 and below (and is used only as a workaround for IE specific bug), so testing this feature in non-IE browsers doesn't have sense and only wastes time.
Probably, but the point of feature testing is that you don't trust the UA string and want to see for yourself.
I'm closing this for now. I'm open to a reassessment of our feature testing strategy; but if we want a change in policy, best to do it all at once rather than piecemeal. Keep this in mind for 1.8.