test($compile): check no memory leak with coexisting element transcludes

If an element contains two "element" transcludes then the initial clone
consists of only comment nodes.  The concern was that this meant that
the transclude scopes would not be cleaned up.

But it turns out that in the case that there are only comments then the
scope is never attached to anything so we don't need to worry about cleaning
it up.

Later if a concrete element is created as part of the transclude then these
elements will have destroy handlers.