CSHARP-2842: Fix AggregateExplain test.

Author: DmitryLukyanov

tests/MongoDB.Driver.Legacy.Tests/MongoCollectionTests.cs

@@ -21,7 +21,6 @@
 using MongoDB.Bson;
 using MongoDB.Bson.IO;
 using MongoDB.Bson.Serialization.Attributes;
-using MongoDB.Driver;
 using MongoDB.Driver.Builders;
 using MongoDB.Driver.Core;
 using MongoDB.Driver.GeoJsonObjectModel;
@@ -31,7 +30,6 @@
 using MongoDB.Driver.Core.TestHelpers.XunitExtensions;
 using MongoDB.Driver.Core.Clusters;
 using MongoDB.Driver.Core.Misc;
-using MongoDB.Driver.TestHelpers;
 using MongoDB.Driver.Core.Events;
 using MongoDB.Driver.Legacy.Tests;
 
@@ -161,7 +159,8 @@ public void TestAggregateExplain()
                     }
                 });
 
-                Assert.True(result.Response.Contains("stages"));
+                var response = result.Response;
+                Assert.True(response.Contains("stages") || response.Contains("queryPlanner"));
             }
         }