tests: demand_paging/mem_map: move malloc config to prj.conf

Both tests set CONFIG_COMMON_LIBC_MALLOC_ARENA_SIZE=0 so extract
it to prj.conf. The tests require malloc arena size to be 0 to
run successful. By refactoring the config, it is now able to
with west build and still passing.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>