cmake: use fixture for preparing venv

cmake: use fixture for preparing venv

this change should allow us to decouple "ninja tests" from "ctest".
in other words, we can just run

ctest -R run-tox-python-common -V

without running "ninja tests" first. before this change

${name}-venv is added as a dependency of "tests" target.

after this change,

setup-venv-for-${name} is added as a test, which is in turn a test of
run-tox-${name}, so we can just

ctest -R run-tox-${name}

now for preparing the venv and then testing the tox test of ${name}.

Signed-off-by: Kefu Chai <kchai@redhat.com>