Fix iOS and Android test execution

Cause all but desktop targets to reference dotnet execution assemble so that it gets packaged up with the mobile app, provided they include this line in their entrypoint method:

this.AddExecutionAssembly(typeof(SkippableFactDiscoverer).Assembly);

Author: AArnott

src/Xunit.SkippableFact.NuGet/Xunit.SkippableFact.NuGet.nuproj

@@ -57,9 +57,15 @@
   </Target>
   <Target Name="MovePlatformExecutionAssemblies" AfterTargets="GetPackageFiles" DependsOnTargets="GetPackageFiles">
     <ItemGroup>
+      <!-- MOVE desktop execution assembly to the build folder. -->
       <PackageFile>
         <TargetPath Condition=" '%(FileName)' == 'xunit.skippablefact.desktop' ">build\%(FileName)%(Extension)</TargetPath>
-        <TargetPath Condition=" '%(FileName)' == 'xunit.skippablefact.dotnet' ">build\%(FileName)%(Extension)</TargetPath>
+      </PackageFile>
+
+      <!-- COPY dotnet assembly to the build folder as well. -->
+      <PackageFile Include="@(PackageFile)"
+                   Condition=" '%(FileName)' == 'xunit.skippablefact.dotnet' ">
+        <TargetPath>build\%(FileName)%(Extension)</TargetPath>
       </PackageFile>
     </ItemGroup>
   </Target>