Fix Create_temporal_table_with_period_columns_not_hidden test

Period columns are shadow properties at the model level regardless of the
HIDDEN SQL flag, so the IColumn collection only exposes Id and Name. The
HIDDEN flag affects emitted SQL only, not table.Columns.

Author: m-x-shokhzod

test/EFCore.SqlServer.FunctionalTests/Migrations/MigrationsSqlServerTest.TemporalTables.cs

@@ -92,9 +92,7 @@ await Test(
                 Assert.Collection(
                     table.Columns,
                     c => Assert.Equal("Id", c.Name),
-                    c => Assert.Equal("Name", c.Name),
-                    c => Assert.Equal("SystemTimeEnd", c.Name),
-                    c => Assert.Equal("SystemTimeStart", c.Name));
+                    c => Assert.Equal("Name", c.Name));
             });
 
         AssertSql(