MODE-1878 Corrected determination of correct SNS child node definitions

When a node with a child is persisted, and then a new child node with the same name as the existing child is added to the parent, and when the parent node type contains more than one applicable residual child node definition, ModeShape incorrected determined that no child node definition could be found for the new SNS child.

Two new test cases were added to replicate this scenario, and the validation logic was corrected.