readd accidentally removed thread library

Author: adelinowona

tests/MongoDB.Driver.Tests/Search/AtlasSearchTests.cs

@@ -16,6 +16,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Threading;
 using FluentAssertions;
 using MongoDB.Bson;
 using MongoDB.Bson.Serialization;