[Mono.Android-Tests] Ignore temporary HTTP errors in async test methods.

[jpobst]

In the never-ending saga of trying to ignore temporary HTTP errors in Mono.Android-Tests test methods, our latest attempt uses Assert.Ignore. Behind the scenes, Assert.Ignore breaks out of a test by throwing NUnit.Framework.IgnoreException. However our very dated test runner doesn't seem to handle this exception properly when in an async Task test method, resulting in this test failure:

System.AggregateException : AggregateException_ctor_DefaultMessage (Ignoring network/server failure: ServiceUnavailable)
----> NUnit.Framework.IgnoreException : Ignoring network/server failure: ServiceUnavailable
   at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean )
   at System.Threading.Tasks.Task.Wait(Int32 , CancellationToken )
   at System.Threading.Tasks.Task.Wait()
   at NUnit.Framework.Internal.AsyncInvocationRegion.AsyncTaskInvocationRegion.WaitForPendingOperationsToComplete(Object invocationResult)
   at NUnit.Framework.Internal.Commands.TestMethodCommand.RunAsyncTestMethod(TestExecutionContext context)
--IgnoreException
   at NUnit.Framework.Assert.Ignore(String message, Object[] args)
   at NUnit.Framework.Assert.Ignore(String message)
   at Xamarin.Android.NetTests.AndroidHandlerTestBase.EnsureSuccessStatusCode(HttpResponseMessage response)
   at Xamarin.Android.NetTests.AndroidMessageHandlerTests.ServerCertificateCustomValidationCallback_Redirects()

For the 2 async Task tests we have, instead check if the HTTP status code indicates we should ignore the test, and if so, just exit the test early. It will show up as "Passed" instead of "Ignored", which isn't perfect, but better than "Failed".