Audit the existing System.Numerics tests to validate the inputs are not all the same

[tannergooding]

It was discovered that dotnet/corefx#31779 introduced a regression due to the values for certain tests being the same between the LHS and RHS inputs.

We should audit the tests to ensure that they are testing unique inputs as well so that core functionality isn't missed and more regressions aren't introduced.

[aashikgowda]

I have a PR for this issue. Changed tests in System.Numeric.Tests.Matrix3x2Tests and System.Numeric.Tests.Matrix4x4Tests to have unique inputs for matrix add, subtract and multiplication tests.