DERBY-6181; fix AutoloadTest to support running concurrently when different base ports are specified.
  backporting revision 1480462 and 1481448 from trunk, reinstating the test but passing on
  the portnumber if set with -Dderby.tests.basePort.


git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/branches/10.9@1481462 13f79535-47bb-0310-9956-ffa450edef68