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.