tests: kernel: userspace: restrict gap-filling test to valid boards

We need to exclude the .gap_filling test from running on
ARMv8-M platforms with 8 MPU regions available, since the
userspace test defines and uses a memory domain whose number
of partitions exceed the maximum number of permitted partitions
in ARMv8-m SoCs with MPU_GAP_FILLING=y.

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