test($rootScope): add assertion to test ensuring that NaN -> NaN does not throw

https://github.com/angular/angular.js/commit/fb6062fb9d83545730b993e94ac7482ffd43a62c implements a
fix for NaN values causing $watchCollection to throw an infdig error. This change updates the test
by adding an assertion which explains what is actually being tested a bit better, and may also
provide better information in the event that the test ever fails.

Closes #6758