Tests for Ember metal watch/unwatch on "length" properties

Tests address TODO in commit @49e90832ac28d5e14aa3cdc0353e9ad9f3772757

Watching the 'length' property on an object should work like any other property. However, the 'length' property of an array is special and cannot be correctly observed like an object property.

These new tests fail prior to the fixes implemented in @49e90832ac28d5e14aa3cdc0353e9ad9f3772757 and pass with the fixes in place.