tests/docker: add python3-venv dependency

tests/docker: add python3-venv dependency

Several debian-based tests need the python3-venv dependency as a
consequence of Debian debundling the "ensurepip" module normally
included with Python.

As mkvenv.py stands as of this commit, Debian requires EITHER:

(A) setuptools and pip, or
(B) ensurepip

mkvenv is a few seconds faster if you have setuptools and pip, so
developers should prefer the first requirement. For the purposes of CI,
the time-save is a wash; it's only a matter of who is responsible for
installing pip and when; the timing is about the same.

Arbitrarily, I chose adding ensurepip to the test configuration because
it is normally part of the Python stdlib, and always having it allows us
a more consistent cross-platform environment.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230511035435.734312-12-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>