[tests] Use simple assertion for ManagedPeer test (#1435)

Avoid CollectionAssert.Contains so the test continues to compile in dotnet/android, where only the simple NUnit assertions are available.

### Add LINQ import for ManagedPeer test

The simple assertion uses Enumerable.Contains, so add the explicit System.Linq import for projects without implicit usings.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: simonrozsival

tests/Java.Interop-Tests/Java.Interop/JniRuntimeTest.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Reflection;
 using System.Collections.Generic;
+using System.Linq;
 using System.Threading;
 
 using Java.Interop;
@@ -70,7 +71,7 @@ public void CreateJavaVMWithNullBuilder ()
 		public void BuiltInSimpleReferenceMap_ContainsManagedPeerByDefault ()
 		{
 			var types = JniRuntime.CurrentRuntime.TypeManager.GetTypes (new JniTypeSignature (ManagedPeer.JniTypeName));
-			CollectionAssert.Contains (types, typeof (ManagedPeer));
+			Assert.IsTrue (types.Contains (typeof (ManagedPeer)));
 		}
 
 #if !__ANDROID__