If your tests are in separate folder then add an __init__.py file to the test folder to make it a package. This simple step will ensure that the test folder is recognized as a package and can be easily imported.
To add an __init__.py file, simply create a new file named __init__.py in the test folder where the test files are stored. You can leave this file empty, or include any necessary initialization code for your tests.
Execute the mutatest.yml workflow manually, to discover opportunities of test-improvements.