add tests on python side

[dance858]

Once we add bindings and want to make a new sparsediffpy release, let's add one test for diag_mat and one test for upper_tri on the python side.

Related PR: #54

[Transurgeon]

I will make a PR with more tests.
I also encountered the following while testing:

• left and right matmul with Python 1d arrays (vectors).
• vector vector matmul (need to double check this)
• parametric expression in left matmul (hstack (p1, p2))