[mod] use tests/unit/settings/test_settings.yml in unit tests

In unit tests settings from

    searx/settings.yml

and the user settings from:

    unit/settings/test_settings.yml

are used.  In the latter, settings can be activated that are needed in the unit
test but should not activated by default in production.

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