use our new dupe item error constant in tests. --ch11l050--
Change isinstances tests to new form. --ch11l051--
split out 3 tests for dupe item view validation. --ch11l052--
rename old test methods for clarity. --ch11l053--
Revert "rename old test methods for clarity. --ch11l053--"

This reverts commit 060682ed434508fb4a56800950e3f0acd5d246e7.
Revert "split out 3 tests for dupe item view validation. --ch11l052--"

This reverts commit 5799c9d4314877dc150d7c1aad9c128c294f2742.
use new ExistingListItemForm in view. --ch11l056--