Mk/Uses/python.mk: Add test support for pytest, unittest and unittest2

[sunpoet]

Summary

This patch adds "make test" support for pytest, unittest and unittest2.

The pytest part was derived from USES=pytest (D34693). All user-facing variables are kept unchanged. USES=pytest can be simply changed to USE_PYTHON=pytest after this patch lands.

Test Plan

Run 'make test'

[sunpoet]

Fix a typo

[sunpoet]

Preliminary support for "make test" for nose, nose2, pytest, unittest and unittest2 has been added to the ports tree on Jun 4, 2022.