Fix hard-coded timestamp in unit tests (#7413)

Several unit tests started failing after the new year due to a hard-coded timestamp that was supposed to represent a valid future time, but is now in the past. This change updates the test code to generate a timestamp that is one year in the future relative to DateTime.Now. I confirmed the unit tests pass once more.

## Azure IoT Edge PR checklist:

edge-util/test/Microsoft.Azure.Devices.Edge.Util.Test.Common/TokenHelper.cs

@@ -9,7 +9,7 @@ namespace Microsoft.Azure.Devices.Edge.Util.Test.Common
     public static class TokenHelper
     {
         public static string CreateSasToken(string resourceUri, string key = null, bool expired = false) =>
-            CreateSasToken(resourceUri, expired ? new DateTime(2010, 1, 1) : new DateTime(2025, 1, 1), key);
+            CreateSasToken(resourceUri, expired ? new DateTime(2010, 1, 1) : DateTime.Now.AddYears(1), key);
 
         public static string CreateSasToken(string resourceUri, DateTime expiryTime, string key = null)
         {