Add msbuild regression test for native references with spaces

We found a last minute regression in 15.8 that needs tests written to prevent future regressions:

https://devdiv.visualstudio.com/DevDiv/_workitems/edit/649776

### Steps to Reproduce

1. Create a new XI project
2. Add a native reference with a space in the path
3. Build with 15.8 and notice failure.

### Expected Behavior
No regression from 15.7 in being build-able

### Actual Behavior

`/Users/donblas/Projects/My App With Spaces/MTOUCH: Error MT0007: The root assembly 'Lib\' does not exist (MT0007) (MyAppWithSpaces)`

Build Failed

### Environment
15.8 in Beta vs Stable.

### Build Logs

https://gist.github.com/chamons/c49975dfc6d1f29db8e680f61fee48cd

### Example Project (If Possible)

[My App With Spaces.zip](https://github.com/xamarin/xamarin-macios/files/2210936/My.App.With.Spaces.zip)