Let the .NET core/standard WebHostBuilder use a random port (#417)

* wip

* code reformat

test/WireMock.Net.Tests/ObservableLogEntriesTest.cs

@@ -96,7 +96,7 @@ namespace WireMock.Net.Tests
             var listOfTasks = new List<Task<HttpResponseMessage>>();
             for (var i = 0; i < expectedCount; i++)
             {
-                Thread.Sleep(10);
+                Thread.Sleep(50);
                 listOfTasks.Add(http.GetAsync($"{server.Urls[0]}{path}"));
             }
             var responses = await Task.WhenAll(listOfTasks);