- expand the check to determine if a selectable column is embedded

- expand the check to determine if a selectable column is embedded
in the corresponding selectable to take into account clones
of the target column.  fixes [ticket:2419]
- have _make_proxy() copy out the _is_clone_of attribute on the
new column so that even more corresponding_column() checks
work as expected for cloned elements.
- add a new test fixture so that mapped tests can be specified
using declarative.