Use next_rails namespace on spec tests.

[JuanVqz]

Description

There were some unnecessary require_relative calls in the spec files, so I removed them and organized the test suite according to the classes in the library folder.

Motivation and Context

When creating additional classes within the NextRails namespace, it became challenging to utilize require_relative calls for loading the classes in spec files.

How Has This Been Tested?

The test suite should still pass.

Screenshots:

I will abide by the code of conduct