Fix test TearDown when requirements ignore tests

Author: alex-kulakov

Orm/Xtensive.Orm.Tests.Sql/IndexTest.cs

@@ -50,7 +50,9 @@ protected override void TestFixtureSetUp()
 
     protected override void TestFixtureTearDown()
     {
-      if (Connection.UnderlyingConnection!=null && Connection.State != System.Data.ConnectionState.Open) {
+      if (Connection is null)
+        return;
+      if (Connection.State != System.Data.ConnectionState.Open) {
         base.TestFixtureTearDown();
         return;
       }

Orm/Xtensive.Orm.Tests.Sql/SqlServer/ExtractorTest.cs

@@ -23,6 +23,9 @@ public class ExtractorTest : SqlTest
 
     protected override void TestFixtureTearDown()
     {
+      if (Connection is null)
+        return;
+
       foreach (var dropOperation in dropOperations) {
         try {
           _ = ExecuteNonQuery(dropOperation);