[benweru]

What and why

make test used POSIX [ -x ... ] in its recipe. With plain make on Windows the recipe runs under cmd and fails. This PR moves the interpreter choice into make itself, the way make dataset already does it. RUN_PY is the venv python if it exists, otherwise $(PYTHON), and test, test-cfs and dataset all use it.

How it was verified

Before, on main, Windows 11 with scoop make and no SHELL override:

make : -x was unexpected at this time.
make: *** [Makefile:18: test] Error 255

After, same machine, plain make test:

.venv/Scripts/python -m pytest -q -m "not cfs and not docker and not linux_i2c"
................................s....sss...............................  [100%]
67 passed, 4 skipped, 15 deselected in 5.56s

Fallback when there's no venv (make -n test VENV=nonexistent):

python -m pytest -q -m "not cfs and not docker and not linux_i2c"

Linux CI runs make test. See the check on this PR.