[fix] test.robot - do not use environment from ./utils/brand.env

Some defaults in the settings.yml are taken from the environment.
By example;

The manage scripts sources the ./utils/brand.env and sets SEARX_PORT
environment.  This enviroment *wins over* any settings in a YAML file.
Whe we run a::

    make test.robot

The searx/settings_robot.yml is used, in this file the server settings are::

    server:
      port: 11111
      bind_address: 127.0.0.1

To get in use of the 'port: 11111' we have to unset the SEARX_PORT environment
which was sourced before.

The function buildenv.unset_env() can be called in all use cases where the
enviroment from ./utils/brand.env is not wanted.  ATM it unset the enviroment
variables::

    unset GIT_URL
    unset GIT_BRANCH
    unset SEARX_URL
    unset SEARX_PORT
    unset SEARX_BIND_ADDRESS

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>