[tests] improve assertions in CancelRequestViaProxy()

[jonathanpeppers]

This test is flaky, but it currently has a terrible error message as "1":

Mono.Android.NET_Tests, Xamarin.Android.NetTests.AndroidMessageHandlerIntegrationTests.HttpClientIntegrationTestBase.CancelRequestViaProxy / Debug
Error message
   1
Stack trace
   at Xamarin.Android.NetTests.HttpClientIntegrationTestBase.CancelRequestViaProxy() in /Users/runner/work/1/s/tests/Mono.Android-Tests/Mono.Android-Tests/Xamarin.Android.Net/HttpClientIntegrationTests.cs:line 166
   at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args)
   at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)

Let's improve the assertion message, in general, and that might help diagnose the flakiness in the future.

This shouldn't alter the behavior of the test, just the assertion message.