tests: add quiet-venv-pip macro

Factor out the "test venv pip" macro; rewrite the "check-venv" rule to
be a little more compact. Replace the "PIP" pseudo-command output with
"VENVPIP" to make it 1% more clear that we are talking about using pip
to install something into a venv.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220526000921.1581503-6-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>