Use DirectConnection to enable tests explicitly connecting to a replica set

Author: ajcvickers

tests/MongoDB.Driver.TestHelpers/Core/XunitExtensions/RequireServer.cs

@@ -171,17 +171,9 @@ public RequireServer Supports(params Feature[] features)
         }
 
         public RequireServer SupportsCausalConsistency()
-            // Determine if we are running against Atlas local by determining if the Atlas connection
-            // string is the same as the regular connection string when the cluster is standalone.
-            // This means that if someone has setup Atlas specific tests locally, then these tests will
-            // also run, otherwise we will skip them on a standalone cluster.
             => CoreTestConfiguration.Cluster.Description.Type == Clusters.ClusterType.Standalone
-               && string.Equals(
-                   CoreTestConfiguration.ConnectionString.ToString(),
-                   Environment.GetEnvironmentVariable("ATLAS_SEARCH_URI"),
-                   StringComparison.Ordinal)
+               && CoreTestConfiguration.ConnectionString.DirectConnection
                 ? this
-                // If the server is not standalone, then use the previous logic.
                 : ClusterTypes(Clusters.ClusterType.Sharded, Clusters.ClusterType.ReplicaSet).SupportsSessions();
 
         public RequireServer SupportsSessions()