server/tests.at: Preserve output in `server communication' tests.

server/tests.at: Preserve output in `server communication' tests.

The test runs with multiple sets of party keys, and used to delete the
party directories before each run, leaving only the output from the
last.  Instead, rename the party directories when we're done.

(I'd just create the directories with the right names in advance, but
the script uses the directory names as parts of shell variable names,
and I don't want to make the directory names ugly to allow that.)