ADD_PYTHON_UNIT_TEST: allow multiple paths in argn (#240)

Multiple paths can be set in PYTHONPATH, but they have to be joined by
`os.pathsep`(":" on unix, ";" on windows)
https://docs.python.org/3/using/cmdline.html#envvar-PYTHONPATH

This commit fix the concatenation with any pre-existing environment
variables, and allows to set several folders (relative to the project
build tree) inside this PYTHONPATH.

eg. if you build a module in src/ and a test module in tests/, you can
import them both in a test with:
ADD_PYTHON_UNIT_TEST("test-python" "tests/main.py" "src" "tests")