[tests] assert that Designer Assembly is not packaged. (#8467)

We had a worry that the `_Microsoft.Android.Resource.Designer.dll` was
being auto included in NuGet packages.

Update the `XASdkTests.DotNetPack()` test to assert that
`_Microsoft.Android.Resource.Designer.dll` is not included in the
generated `.nuget` file.

Spoiler... turns out it wasn't :D

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/XASdkTests.cs

@@ -141,6 +141,7 @@ public String Say (String quote) {
 			nupkg.AssertDoesNotContainEntry (nupkgPath, "content/nopack.aar");
 			nupkg.AssertDoesNotContainEntry (nupkgPath, $"contentFiles/any/{dotnetVersion}-android{apiLevel}.0/nopack.aar");
 			nupkg.AssertContainsEntry (nupkgPath, $"lib/{dotnetVersion}-android{apiLevel}.0/baz.aar");
+			nupkg.AssertDoesNotContainEntry (nupkgPath, $"lib/{dotnetVersion}-android{apiLevel}.0/_Microsoft.Android.Resource.Designer.dll");
 		}
 
 		static readonly object[] DotNetTargetFrameworks = new object[] {