twister: extend --force-platform to skip platform_allow options

In many of test specifications yaml files (testcase.yaml, sample.yaml)
section `platform_allow` is added. This change allows to test some
scenarios on platforms, that are not added yet to platform allow
list (or are not going to be added for some reasons).

Signed-off-by: Grzegorz Chwierut <grzegorz.chwierut@nordicsemi.no>