Fix matrix_inverse_tril_test handling the inverse of 0

The inverse of 0 is undefined, so when computing the inverse of a lower
triangular numpy array, we need to ensure the zeros in the upper
parts of the triangle remain as zero. After doing the inverse
of an array, iterator over the array and set the values above the
main diagonal to zero.
See: https://github.com/numpy/numpy/issues/11445
Fixes: #20013