make server pool test wait for membership to catch up

before making assertions, as resize happens in bg (whereas start is immediate)

fixes http://brooklyn.builds.cloudsoftcorp.com/job/Brooklyn-Master-Build/io.brooklyn$brooklyn-software-base/418/testReport/junit/brooklyn.entity.pool/ServerPoolTest/testResizingPoolUp/

(confirmed by inserting a sleep in `ServerPoolImpl.serverAdded` which triggers the failure before this fix is applied)

maybe we want instead to change `resize` to wait for membership to catch up?  i wasn't sure...