tests: mem_protect: merge two duplicate tests

The tests test_mem_part_auto_determ_size and
test_mem_part_auto_determ_size_per_mpu are supposed to
just be checking the construction of automatic memory
partitions.

test_mem_part_auto_determ_size had a bunch of extraneous
stuff covered by other test cases and reserved three
different thread stacks.

These two tests have been drastically simplified and
combined.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>