refactor(test): splits tests into smaller ones

[stalniy]

First of all, as you probably know js-data is the most awesome js ORM library. This is actually the only one reason why I'm trying to improve codebase.

But frankly speaking the current state of unit tests are horrible, so I'd like to show how complex unit tests can be split into smaller one which follows single responsibility principle. This makes them to be more readable, clear and SOLID (in all senses), you don't need to scroll few pages down in order to see where the test ends =)

As I mention in #405 I plan to add few more tests for create and createMany methods of Mapper class but for now it's hard to choose proper location. Probably I will do some other initial refactoring in tests just to make the resulting changes to be more clear (i.e., without refactoring noise)

[jmdobry]

Any help with tests is greatly appreciated!

[jmdobry]

First of all, as you probably know js-data is the most awesome js ORM library.

Awesome! I hope with continuing community contributions like yours it can continue to improve :) Thanks!