Update Unit Testing documentation

[mexitalian]

The previous import location threw an error for me as it couldn't be found.

[djhi]

The react-admin package just re-export what ra-core exports. So both imports should work

[Kmaschta]

I somewhat agree with that PR: ra-core isn't documented (yet) and it might be confusing to the early adopters why we require that package in a simple unit test example.

[djhi]

@fzaninotto What do you think about it? I agree with @Kmaschta

[fzaninotto]

Thanks!