[Tests] Fix TryGetSupportedInterfaces when we have values. Fixes #2273 (#9276)

* [Tests] Fix TryGetSupportedInterfaces when we have values. Fixes #2273

Make the test check against the value returned by the native API rather
than expect the result to always be null.

Fixes https://github.com/xamarin/maccore/issues/2273

* Address reviews.

tests/monotouch-test/SystemConfiguration/CaptiveNetworkTest.cs

@@ -91,7 +91,6 @@ namespace MonoTouchFixtures.SystemConfiguration {
 #else
 			status = CaptiveNetwork.TryGetSupportedInterfaces (out var ifaces);
 			Assert.AreEqual (StatusCode.OK, status, "Status");
-			Assert.IsNull (ifaces, "Null Interfaces");
 #endif // __TVOS__
 		}
 #endif