Add additional tests to verify _is_clone_of proxy level link
Add additional tests to verify _is_clone_of proxy level link

The commit message in 896d47f318c5c27620fd6da is not accurate,
we do in fact still need _is_clone_of when a clone of a column is
created so that we can implement _cloned_set().  Additionally,
the linkage of _is_clone_of within make_proxy() also suits
an additional use case that seems to be related to [ticket:2419].
Adjust one of the tests which likely got changed within 1.4's
refactoring of Select to test this correctly.

Change-Id: I124c7c6b02498e2dfad9797816df42a5b6f91901

graingert/sqlalchemy,zzzeek/sqlalchemy,j5int/sqlalchemy,wujuguang/sqlalchemy,monetate/sqlalchemy,graingert/sqlalchemy,wujuguang/sqlalchemy,sqlalchemy/sqlalchemy,monetate/sqlalchemy,j5int/sqlalchemy