tests: exclude twr_ke18f platform from several user mode tests

Several user mode tests cannot run on twr_ke18f because
either the platform does not have a sufficient number of
MPU regions required for the tests, or, the tests also
require HW stack protection (which has been, by default,
excluded in user mode tests for twr_ke18f board). We
excluded the board from all those tests.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>