unittest does not find tests as it confuses `tests/unit` for a module

[IshaanDesai]

In some cases it is observed that unittest is unable to reach the test even when the file path is specified to it in the following form:

python -m unittest tests/unit/test_diffusion2d_functions.py

It is unclear why this happens, but a possible solution is to add a blank __init__.py file in the tests/ folder.