[tests] add SourceGen.UnitTests to CI (#21889)

Context: #21725

In #21725 we noticed the source generator unit tests (for `*.xaml.g.cs`) don't run on CI.

Whoops!

Author: jonathanpeppers

eng/cake/dotnet.cake

@@ -222,6 +222,7 @@ Task("dotnet-test")
             "**/Controls.Core.UnitTests.csproj",
             "**/Controls.Core.Design.UnitTests.csproj",
             "**/Controls.Xaml.UnitTests.csproj",
+            "**/SourceGen.UnitTests.csproj",
             "**/Core.UnitTests.csproj",
             "**/Essentials.UnitTests.csproj",
             "**/Resizetizer.UnitTests.csproj",