fixing H2 unit test failures

the fix would ensure we keep at least 2 open connections during the tests, so that
when an (intentional) exception happens in DB operation, closing the connection
doesn't kill the in-memory db

git-svn-id: https://svn.apache.org/repos/asf/cayenne/main/trunk@1546153 13f79535-47bb-0310-9956-ffa450edef68
fixing H2 unit test failures

not cleaning up the DB properly

git-svn-id: https://svn.apache.org/repos/asf/cayenne/main/trunk@1546154 13f79535-47bb-0310-9956-ffa450edef68