fix(robot_cli): support running without xvfb-run

custom certificate authorities (declared using either `$CUSTOM_CA_CERTS` or `$DEFAULT_CA_CERTS`). If you need it (need to call a server

with SSL certificate issued by a non-official certificate authority), then you'll have to build your own Docker image that either embeds

the required CA certificates, or that runs as `root` (thus the template will be able to install custom certificate authorities).

* The template runs [XVFB](https://www.x.org/releases/X11R7.6/doc/man/man1/Xvfb.1.xhtml) with `-screen` parameter

* The template runs [XVFB](https://www.x.org/releases/X11R7.6/doc/man/man1/Xvfb.1.xhtml) if available with `-screen` parameter

  `$ROBOT_SCREEN_WIDTHx$ROBOT_SCREEN_HEIGHTx$ROBOT_SCREEN_COLOUR_DEPTH` to run tests.

  When no xvfb-run command is available, robot/pabot are run directly (usefull for slim images when no playwright/browser is involved in tests, example : when testing REST APIs with Requests Library)

* Depending on the `ROBOT_THREADS` value, the template runs either [`robot`](https://robotframework.org/robotframework/latest/RobotFrameworkUserGuide.html#all-command-line-options)

  or [`pabot`](https://pabot.org/) (if `ROBOT_THREADS` > 1).

* Tests are run automatically on `master` and `develop` branches, and manually on others (may be overridden setting the

    then

      export ROBOT_SYSLOG_LEVEL=DEBUG

    fi

    # Build xvfb command if available

    if command -v xvfb-run > /dev/null

    then

      XVFB_PREFIX=(xvfb-run --server-args="-screen 0 ${ROBOT_SCREEN_WIDTH}x${ROBOT_SCREEN_HEIGHT}x${ROBOT_SCREEN_COLOUR_DEPTH} -ac")

    else

      XVFB_PREFIX=()

    fi

    if [ "$ROBOT_THREADS" -eq 1 ]

    then

      # shellcheck disable=SC2086

      xvfb-run \

        --server-args="-screen 0 ${ROBOT_SCREEN_WIDTH}x${ROBOT_SCREEN_HEIGHT}x${ROBOT_SCREEN_COLOUR_DEPTH} -ac" \

      "${XVFB_PREFIX[@]}" \

        robot $base_url_opt "$@"

    else

      # shellcheck disable=SC2086

      xvfb-run \

            --server-args="-screen 0 ${ROBOT_SCREEN_WIDTH}x${ROBOT_SCREEN_HEIGHT}x${ROBOT_SCREEN_COLOUR_DEPTH} -ac" \

      "${XVFB_PREFIX[@]}" \

            pabot \

            ${TRACE+--verbose} \

            --processes "$ROBOT_THREADS" $PABOT_OPTIONS $base_url_opt "$@"