tests: add "TESTS_PYTHON" variable to Makefile

This is a convenience feature: $(PYTHON) points to the Python executable
we were instructed to use by the configure script. We use that Python to
create a virtual environment with the "check-venv" target in
tests/Makefile.include.

$(TESTS_PYTHON) points to the Python executable belonging to the virtual
environment tied to the build. This Python executable is a symlink to
the binary used to create the venv, which will be the version provided
at configure time.

Using $(TESTS_PYTHON) therefore uses the $(PYTHON) executable, but with
paths modified to use packages installed to the venv.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220526000921.1581503-3-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>