ResolveRelativePath Fixes

This PR builds on the work @lordoaty did in https://github.com/mono/MonoGame/pull/3073.

It adds fixes for unix-like systems and includes a unit test that hopefully covers the gamut of possible paths we can get.

Fixes https://github.com/mono/MonoGame/issues/3064.
Fixes https://github.com/mono/MonoGame/issues/3041.
It would also be clearer to use `@"\\"` in place of `"\\\\"` in this case.