Sync & centralize nuget package versions

Context: https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=6904461&view=logs&j=cac0e8d3-0ef5-5d2b-b57e-e8fde7204df3&t=d8447b4e-561a-55bf-52c2-03d87ae8f26c

The **run Smoke MSBuild Tests - Windows Build Tree** step is failing:

```
NUnit.Engine.NUnitEngineException : An exception occurred in the driver while loading tests.
  ----> System.IO.FileLoadException : Could not load file or assembly 'nunit.framework, Version=3.13.2.0, Culture=neutral, PublicKeyToken=2638cd05610744eb' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)
--NUnitEngineException
An exception occurred in the driver while loading tests.
…
```

No idea why this would be failing; it doesn't make sense to me.

My only *guess* is that we have two different nunit.framework
package versions being pulled in -- which is TRUE -- and that is
screwing things up -- which I'm not sure I can understand.

Regardless, update the NUnit-related package versions that
`Xamarin.Android.Tools.Aidl-Tests.csproj` uses so that it is
consistent with the other package versions in the repo.

Author: jonpryor

tests/Xamarin.Android.Tools.Aidl-Tests/Xamarin.Android.Tools.Aidl-Tests.csproj

@@ -7,21 +7,12 @@
   </PropertyGroup>
 
   <Import Project="..\..\Configuration.props" />
+  <Import Project="..\..\build-tools\scripts\NUnitReferences.projitems" />
 
   <PropertyGroup>
     <OutputPath>..\..\bin\Test$(Configuration)</OutputPath>
   </PropertyGroup>
 
-  <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.5.0" />
-    <PackageReference Include="nunit" Version="3.12.0" />
-    <PackageReference Include="NUnit.ConsoleRunner" Version="3.11.1" />
-    <PackageReference Include="NUnit3TestAdapter" Version="3.16.1">
-      <PrivateAssets>all</PrivateAssets>
-      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
-    </PackageReference>
-  </ItemGroup>
-
   <ItemGroup>
     <None Update="TestData/*.txt" CopyToOutputDirectory="PreserveNewest" />
   </ItemGroup>