Bump Touch.Unit (dark mode support) (xamarin#7020)

New commits in spouliot/Touch.Unit:

* xamarin/Touch.Unit@d22825a [TouchRunner] Fix colors for iOS 13 dark mode (xamarin#53)
* xamarin/Touch.Unit@de42aa3 Fallback to "modern" mtouch root if `MONOTOUCH_ROOT` is not set. (xamarin#52)
* xamarin/Touch.Unit@a2e1810 Improve logging for failed TCP connections due to hostname (xamarin#51)

Diff: https://github.com/spouliot/Touch.Unit/compare/1c41c75983ed840e9dc7079b5d4b0c31d1565359..d22825a7d13b3a1e96983d132c9d3cfc5ba9762f