With ports as of r512417,
it seems that testport -P is useless because on several ports I get "@dir /prefix" flagged as leftover, which it isn't - the prefix is normally outside of a port's scope, and flagging a root directory seems pointless to me. One other port is graphics/rawtherapee, but I've not bothered beyond two ports.
[00:00:28] [120amd64-svn] [1/1] Deinstalling bogofilter-1.2.5.r1_1...
[00:00:28] [120amd64-svn] [1/1] Deleting files for bogofilter-1.2.5.r1_1: .......... done
[00:00:28] =>> Checking for extra files and directories
[00:00:28] =>> Error: Files or directories left over:
[00:00:28] @dir /prefix
[00:00:28] =>> This test was done with PREFIX!=LOCALBASE which may show failures if the port does not respect PREFIX.