Work around a HttpListener framework bug

[alanmcgovern]

If HttpListener throws an exception in it's async callback because it's already been disposed, then it's a framework bug and they should fix it. I'm pretty sure i filed it before and it wasn't addressed, so work around it by ignoring the exit code from the test running process and only fail the build if tests have failed.

This is no longer causing red builds:

The active test run was aborted. Reason: Test host process crashed : Unhandled exception. System.ObjectDisposedException: Safe handle has been closed.
Object name: 'SafeHandle'.
   at System.Runtime.InteropServices.SafeHandle.DangerousAddRef(Boolean& success)
   at System.StubHelpers.StubHelpers.SafeHandleAddRef(SafeHandle pHandle, Boolean& success)
   at Interop.HttpApi.HttpCancelHttpRequest(SafeHandle requestQueueHandle, UInt64 requestId, IntPtr pOverlapped)
   at System.Net.HttpListenerContext.ForceCancelRequest(SafeHandle requestQueueHandle, UInt64 requestId)
   at System.Net.HttpListenerContext.Abort()
   at System.Net.HttpResponseStream.EndWriteCore(IAsyncResult asyncResult)
   at System.Net.HttpResponseStream.EndWrite(IAsyncResult asyncResult)
   at System.Net.HttpListenerResponse.NonBlockingCloseCallback(IAsyncResult asyncResult)
   at System.Net.LazyAsyncResult.Complete(IntPtr userToken)
   at System.Net.LazyAsyncResult.ProtectedInvokeCallback(Object result, IntPtr userToken)
   at System.Net.HttpResponseStreamAsyncResult.IOCompleted(HttpResponseStreamAsyncResult asyncResult, UInt32 errorCode, UInt32 numBytes)
   at System.Net.HttpResponseStreamAsyncResult.Callback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* nativeOverlapped)
   at System.Threading._IOCompletionCallback.IOCompletionCallback_Context(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
--- End of stack trace from previous location ---
   at System.Threading._IOCompletionCallback.PerformIOCompletionCallback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* pNativeOverlapped)