gitlab: migrate the minimal tools and unit tests from Travis

These tests are good at shaking out missing stubs which otherwise work
if we have built targets. Rather than create a new job just add the
checks to the existing tools-and-docs build.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210114165730.31607-12-alex.bennee@linaro.org>