[tests] update everything to use Xamarin.Forms 4.3 #4002
                
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I also improved things so it's easier to bump this going forward:
KnownPackages.XamarinForms_LatestStable, so tests won'tneed to change going forward.
Xamarin.Forms.Performance.Integrationnow uses<PackageReference/>. The netstandard library was already.I also removed some missing
Iconfiles from the forms project. Thesedid not even exist.
XamarinFormsPCL_2_3_4_231was also unused, so I removed it.