Refactor RSpec for AnnotateModels (1) (ctran#726)

I refactored and structuralized RSpec test cases of AnnotateModels for readability and scalability because it was too complex to read.

cf. ctran#718 
In this PR, I refactored test cases of some methods in `AnnotateModels`.
I will refactor test cases of other methods in another PR.

Author: nard-tech