tests: userspace/mem_protect: more CONFIG_SYS_HEAP_ALLOC_LOOPS

Due to the constant alloc/free cycles during the stress test,
it is possible that the heap can become fragmented enough to
prevent successful allocation, and thus failing the test.
So we change the CONFIG_SYS_HEAP_ALLOC_LOOPS to a higher value
so the allocation code will try harder to find a space.

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