Mk/Uses/python.mk: Add support for optional 'test' argument

Some ports may need to use Python for their testing suite but otherwise
do not need it at all (ie, not for build or run). This patch adds
support for the test argument to be used in the USES clause, such as
python:3.2+,test. This enables the relevant Python environment and
modifies TEST_DEPENDS as necessary.

For non-Python ports that use Python as their testing suite, add
python:<ver>,test as required to the USES clause.

PR: 205616
Submitted by: Brendan Molloy <brendan+freebsd bbqsrc net>
Reviewed by: mat, miwi, koobs, antoine
Approved by: koobs (python)
Differential Revision: https://reviews.freebsd.org/D4711