tests: mem_protect: fix partition remove test on v8-m with 8 regions

Some ARMv8-M platforms may come with only 8 (instead of 16)
MPU regions. In these platforms, by design, a memory domain
may contain up to 2 application memory partitions, when we
build with MPU_GAP_FILLING support. To be able to test this
valid configuration we slightly modify the test code in the
mem_protect suite, and add-remove the second partition (with
index-1) instead of the third (index-2).

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