tests: Support targeting unit_testing with and without qualifier

For tests that support both targeting unit_testing and
other targets, we check in the cmake code the BOARD variable.
Let's allow users to set this to either of unit_testing
or unit_testing/unit_testing so it behaves like for other
tests.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>