The test should be set up so that neither Ranma nor Hikaru realizes that it is 

a >test.

I agree. Also, using Nabiki to convince Hikaru to kill Ranma isn't strong 
enough. Think about, does she have any reason to listen to Nabiki? It would be 
far more effective if it was someone close to her like Nodoka to Ranma.
Here's a suggestion for a better test. Put Ranma and Hikaru in a situation 
where only one of them can survive, and the survivor gets the body.