tests: silence pip upgrade warnings during venv creation

Turn off the nag warning coaxing us to upgrade pip. It's not really that
interesting to see in CI logs, and as long as nothing is broken --
nothing is broken.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220526000921.1581503-5-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>