Tests for `homogeneous_lde`

I added two additional tests (very simple ones). Now I remember why I included `Find solutions in negative integers if possible` as a to-do in the [original PR](https://github.com/sympy/sympy/pull/7241). The algorithm we are using can't find solutions in negative integers. For example, the equation `2x + 3y = 0`  has a solution (either `{3, -2}` or `{-3, 2}`) but the algorithm can't find one. Still, the algorithm finds all the positive solutions and can solve a system of linear Diophantine equations efficiently.