initialize connectionURLShutdown in tests

Due to an uninitialized connectionURLShutdown
DriverManager.getConnection(getConnectionURLShutdown()) failed with the
message "The url cannot be null", producing the test output
  "[ERROR] The url cannot be null"
instead of 
  "[INFO] Derby system shutdown."

The error was compensated by the subsequent call to server.shutdown(),
so that the tests were still successful.