Move the tests directory outside of the code directory

[ricardoV94]

Discussed in #6172

^{Originally posted by Armavica October 2, 2022}
Hi,
With the reorganization of the tests complete (#5777), a last question remains: where to place the tests directory. It currently resides inside the pymc code directory:

pymc/
├── benchmarks/
├── docs/
├── pymc/
│   ├── __init__.py
│   ├── model.py
│   ├── ...
│   └── tests/
└── scripts/

but one could argue that it would make more sense to put it at the same level as benchmarks/, docs/ and pymc/:

pymc/
├── benchmarks/
├── docs/
├── pymc/
│   ├── __init__.py
│   ├── model.py
│   └── ...
├── scripts/
└── tests/

This is what is recommended by the official Python packaging guide, by the Pytest good practices guide, the Poetry package management tool as well as by other guides.
However, I have to note that numpy, scipy, and pandas all have tests inside the code directory.

Besides the arguments advanced by the Pytest good practices guide, I can add that I often grep the source code directory to see where or how a function or class is used or defined. With the tests directory where it is currently, a lot of noise is often returned from the tests.

Cost of the transition: probably limited to some imports to change in the test files to reflect the move.

What is your opinion on this?

[ricardoV94]

@Armavica do you want to work on this one?