[Xamarin.Android.Bcl-Tests] Allow Xamarin.Android.NUnitLite.dll path to be overridden (#1030)

This is required for "system" execution on Windows, as there is no
`xbuild-frameworks` folder installed by the Xamarin.Android `.vsix`.
The framework installation location in VS2017 is outlined at:


	C:\Program Files (x86)\Microsoft Visual Studio\Preview\Enterprise\Common7\IDE\ReferenceAssemblies\Microsoft\Framework\MonoAndroid\v1.0'

Instead of hardcoding the path to `Xamarin.Android.NUnitLite.dll`,
let MSBuild sort it out by using the the
`<ResolveAssemblyReference/>` task.

tests/Xamarin.Android.Bcl-Tests/Xamarin.Android.Bcl-Tests.targets

@@ -20,9 +20,11 @@
     <ItemGroup>
       <_Source  Include="@(MonoTestAssembly->'..\..\bin\$(Configuration)\bcl-tests\%(Identity)')" />
     </ItemGroup>
-    <PropertyGroup>
-      <_NUnit>$(XAInstallPrefix)xbuild-frameworks\MonoAndroid\v1.0\Xamarin.Android.NUnitLite.dll</_NUnit>
-    </PropertyGroup>
+    <ResolveAssemblyReference
+        Assemblies="Xamarin.Android.NUnitLite"
+        SearchPaths="$(XAInstallPrefix)xbuild-frameworks\MonoAndroid\v1.0;$(FrameworkPathOverride)">
+      <Output TaskParameter="ResolvedFiles"   PropertyName="_NUnit" />
+    </ResolveAssemblyReference>
     <Exec
         Command="$(RemapAssemblyRefTool) &quot;%(_Source.Identity)&quot; &quot;$(IntermediateOutputPath)%(_Source.Filename)%(_Source.Extension)&quot; nunitlite &quot;$(_NUnit)&quot;"
     />