Update operation result resource items

Adds smtk::mesh::Resource to ImportModel and changes reference item in results attdef to extensible to allow 0 values (to avoid exception from ReferenceItem iterator that's not caught in qtWorkletModel which, in turn, crashes modelbuilder)

Also update tests to check if test project exists before running tests that require it.

Better approach might be to use the resource item's enable flag, but that would required code change in smtk core that could affect all operations returning resources or components.