Add timestamps to test output

A lot of connections are ridiculously slow even to localhost at least in testing...let's timestamp the output (including milliseconds) so we can see what's happening.

StackExchange.Redis.Tests/Helpers/TextWriterOutputHelper.cs

-using System.IO;
+using System;
+using System.IO;
 using System.Text;
 using Xunit.Abstractions;
 
@@ -11,6 +12,13 @@ public class TextWriterOutputHelper : TextWriter
         private readonly ITestOutputHelper Output;
         public TextWriterOutputHelper(ITestOutputHelper outputHelper) => Output = outputHelper;
 
+        public override void WriteLine(string message)
+        {
+            base.Write(DateTime.UtcNow.ToString("HH:mm:ss.fff"));
+            base.Write(": ");
+            base.WriteLine(message);
+        }
+
         public override void Write(char value)
         {
             if (value == '\n' || value == '\r')