Update test cases to use pytest

[amotl]

Problem

The test cases have been adjusted to use pytest instead of unittest. The change happened between MLflow 2.10.2 and 2.11.3.

• Update MLflow from 2.10.2 to 2.11.3 #124

Evaluation

It does not mean a regression in runtime behavior, but it will add drag to future maintenance of the code base.

Solution

Migrate test cases to pytest as well, so updates and changes from upstream can be integrated without further ado.