mat: Improvements to list_test testing behavior

[btracey]

List test works by generating random matrices. We should improve this behavior by a) Having an inner loop where we test on a set of random matrices b) Instead of using rand.Float64() to generate random elements, we should instead use const*rand.NormFloat64()

Fix a) helps to check "nonlinear" functions like Max where the location of the maximum element can vary, and Fix b) ensures we have have matrix values of a variety of signs and sizes. Plausibly we also should have a small change of being 0 as well (5% or something).

[btracey]

ListTwoTest should also fill the entire data slice with values (or possibly NaN) for types like TriDense and SymDense as 0 is a pretty innocuous value with an incorrect access.