add tests for exp primal and sdps

I also added `tol` as an argument since SCS by default isn't accurate enough to 6 decimal digits.

Actually, according to SCS documentation, 
`positive semidefinite cone { X | min(eig(X)) >= 0, X = X^T }`
so it's enforcing symmetry. I just filed an issue here: https://github.com/cvxgrp/scs/issues/31