chore: Use pytester fixture to test the plugin

[edgarrmondragon]

pytester:

Added in version 6.2.

Provides a Pytester instance that can be used to run and test pytest itself.

It provides an empty directory where pytest can be executed in isolation, and contains facilities to write tests, configuration files, and match against expected output.

testdir:

Identical to pytester, but provides an instance whose methods return legacy py.path.local objects instead when applicable.

New code should avoid using testdir in favor of pytester.