Remove user.rs and find a better way to use them

[ozankaymak]

Issue Description

We will not be using user.rs anymore, but we still need their functionalities for testing.

Tasks

Parts of the task:

• Remove user.rs
• Find a better way to include them in the tests

Deadline of the Issue

[ceyhunsen]

It should be noted that all the test code must be macros. Because we use them in both unit and integration tests and one of them must include the file directly (like by specifying relative OS path to it), which was the integration tests. Reason was that we don't want to infiltrate test code into binaries (details: #181).

If we use use statements, integration tests won't compile because include paths were different (crate:: vs clementine_core::). So we decided to use only macros for this kind of stuff. They are in core/src/test_utils.rs. I suggest following the conventions. Pr that introduced this: #375