[tests] Copy and invoke NuGet.exe from subfolder

`NuGet.exe` throws on macOS when it is invoked from the same folder as
`Xunit.NetCore.Extensions.dll`:

    mono bin/TestRelease/NuGet.exe

    VTable setup of type Xunit.NetCore.Extensions.SkippedTestCase failed
    System.TypeLoadException: VTable setup of type Xunit.NetCore.Extensions.SkippedTestCase failed
      at (wrapper managed-to-native) System.RuntimeType.GetPropertiesByName_native(System.RuntimeType,intptr,System.Reflection.BindingFlags,System.RuntimeType/MemberListType)
      at System.RuntimeType.GetPropertiesByName (System.String name, System.Reflection.BindingFlags bindingAttr, System.RuntimeType+MemberListType listType, System.RuntimeType reflectedType) [0x0001b] in <7d0bc631cadd4a07bf74717847e90cfa>:0
      at System.RuntimeType.GetPropertyCandidates (System.String name, System.Reflection.BindingFlags bindingAttr, System.Type[] types, System.Boolean allowPrefixLookup) [0x00010] in <7d0bc631cadd4a07bf74717847e90cfa>:0
      at System.RuntimeType.GetProperties (System.Reflection.BindingFlags bindingAttr) [0x00000] in <7d0bc631cadd4a07bf74717847e90cfa>:0
      at System.ComponentModel.Composition.AttributedModel.AttributedPartCreationInfo+<GetExportMembers>d__32.MoveNext () [0x000eb] in <5e20d4aa8c6c44a89bf1aeb3c06f166c>:0
      at System.Linq.Enumerable.Any[TSource] (System.Collections.Generic.IEnumerable`1[T] source) [0x00015] in <e22351e395f44d7b8c4860fca2dcee71>:0
      ...

Move the copy of `NuGet.exe`into it's own folder to avoid this wierd
behavior when executing `Xamarin.Android.Build.Tests.dll` on macOS.

Author: pjcollins

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Common/XamarinProject.cs

@@ -330,7 +330,7 @@ public virtual void NuGetRestore (string directory, string packagesDirectory = n
 				return;
 
 			var isWindows = Environment.OSVersion.Platform == PlatformID.Win32NT;
-			var nuget = Path.Combine (Root, "NuGet.exe");
+			var nuget = Path.Combine (Root, "nuget", "NuGet.exe");
 			var psi = new ProcessStartInfo (isWindows ? nuget : "mono") {
 				Arguments = $"{(isWindows ? "" : "\"" + nuget + "\"")} restore -Verbosity Detailed -PackagesDirectory \"{Path.Combine (Root, directory, "..", "packages")}\" \"{Path.Combine (Root, directory, "packages.config")}\"",
 				CreateNoWindow = true,

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Xamarin.ProjectTools.csproj

@@ -143,6 +143,7 @@
   </ItemGroup>
   <ItemGroup>
     <Content Include="..\..\..\..\.nuget\NuGet.exe">
+      <Link>nuget\NuGet.exe</Link>
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </Content>
     <Content Include="FrameworkPath.targets">