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.
'./' checks might be useful to ensure that there isn't any strange behaviour there. That's all I can come up with for now.