pytest gathers tests according to a naming convention. By default any file that is to contain tests must be named starting with test_, classes that hold tests must be named starting with Test, and any function in a file that should be treated as a test must also start with test_.
If you rename your test file to test_sorts.py and rename the example function you provide above as test_integer_sort, then you will find it is automatically collected and executed.
This test collecting behavior can be changed to suit your desires. Changing it will require learning about configuration in pytest.