server/tests.at: Work around strace's attempt to overwrite core files.

server/tests.at: Work around strace's attempt to overwrite core files.

When strace sees its child process die from a signal, it sends itself
the same signal in order to propagate the exit status accurately.
Unfortunately, if the child process left a core dump, it gets
overwritten by a useless core dump showing how strace committed suicide.

Work around this by running strace in a subdirectory and getting the
child process to run back in the parent.

Also see Debian bug #656389.