Comment 7 for bug 1816022

But also tempest itself is installed using the requirements from the branch. See the content of install_tempest():

http://git.openstack.org/cgit/openstack-dev/devstack/tree/lib/tempest#n667

# install_tempest() - Collect source and prepare
function install_tempest {
    git_clone $TEMPEST_REPO $TEMPEST_DIR $TEMPEST_BRANCH
    pip_install 'tox!=2.8.0'
    pushd $TEMPEST_DIR
    tox -r --notest -efull
    # NOTE(mtreinish) Respect constraints in the tempest full venv, things that
    # are using a tox job other than full will not be respecting constraints but
    # running pip install -U on tempest requirements
    $TEMPEST_DIR/.tox/tempest/bin/pip install -c $REQUIREMENTS_DIR/upper-constraints.txt -r requirements.txt
    PROJECT_VENV["tempest"]=${TEMPEST_DIR}/.tox/tempest
    popd
}

Even if it's a different tempest environment, the directory is shared (~/.tox/tempest).