The test port allocation had a TOCTOU race where GetFreePort() would
open a listener, grab the port number, then immediately close it.
When called repeatedly, the OS could recycle a just-released port,
causing two services (e.g. Filer and S3) to be assigned the same port.
Replace per-call GetFreePort() with batch AllocatePorts() that holds
all listeners open until every port is obtained, matching the pattern
already used in test/volume_server/framework/cluster.go.
e7fc243ee1