Add Mysql57 version to test framework

Author: alex-kulakov

Orm/Xtensive.Orm.Tests.Framework/StorageProviderVersion.cs

@@ -36,6 +36,7 @@ public static class StorageProviderVersion
 
     public static Version MySql55 = new Version(5, 5);
     public static Version MySql56 = new Version(5, 6);
+    public static Version MySql57 = new Version(5, 7);
 
     public static Version Firebird25 = new Version(2, 5);
     public static Version Firebird30 = new Version(3, 0);