Add test coverage for "Invalid reference while copying itemset answer: Destination already exists!"

[lognaturel]

Software versions

JavaRosa v3.0.2, Collect v1.28.1

Problem description

We have tracked a few instances of a crash in Collect with Invalid reference while copying itemset answer: Destination already exists! We don't know when this might occur.

javarosa/src/main/java/org/javarosa/core/model/instance/FormInstance.java

Line 141 in a42a809

throw new InvalidReferenceException("Destination already exists!", to);