Fix SchemaManagerFunctionalTestCase composite foreign keys test

This PR fixes a bug in the <code>SchemaManagerFunctionalTestCase</code> composite foreign keys test I encountered today. In SQL Server the test case fails because both <code>testListForeignKeys</code> and <code>testListForeignKeysComposite</code> tests create a foreign key with the same name which is not possible for one single DB in SQL Server.
I don't know if this applies to other RDBMS also but this should also fix the others.