Fix nested array set_item test to check array mutation.
Fix nested array set_item test to check array mutation.

As title.

numba/numba,cpcloud/numba,numba/numba,numba/numba,numba/numba,cpcloud/numba,IntelLabs/numba,IntelLabs/numba,IntelLabs/numba,IntelLabs/numba,cpcloud/numba,numba/numba,cpcloud/numba,cpcloud/numba,IntelLabs/numba