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.
I think I found the original author of that line and i'm asking him about it...

https://github.com/mono/MonoGame/commit/ba25ee89633bee5d6582afe4b1e021dc729ab27d#commitcomment-8198375