Mk/Uses/python.mk: Add preliminary support for "make test" for nose, nose2, pytest, unittest and unittest2

This patch adds preliminary support for "make test". It supports nose, nose2,
pytest, unittest and unittest2. Enable this by adding one of the following:

USE_PYTHON=nose
USE_PYTHON=nose2
USE_PYTHON=pytest
USE_PYTHON=unittest
USE_PYTHON=unittest2

The pytest support was derived from USES=pytest. All user-facing variables are
kept unchanged except PYTEST_ARGS which has been changed to generic TEST_ARGS.
USES=pytest can be changed to USE_PYTHON=pytest as follows:

USES=pytest   -> USE_PYTHON=pytest
USES=pytest:4 -> USE_PYTHON=pytest4
PYTEST_ARGS   -> TEST_ARGS