tests/dtest.py: Refer to the home directory via a symlink.

tests/dtest.py: Refer to the home directory via a symlink.

Pathnames during `make distcheck' are remarkably long, even if the
original project path is very short.  Indeed, the previous change which
added the test name into the `testroot' path is enough to push the
server's socket name over the magic 108-byte limit.

As an additional hack, arrange to refer to the home directory via a
symbolic link in `/tmp' (or wherever $TMPDIR points), which should be
much shorter.  The symlink is deleted on test completion, because
otherwise we'll fill up `/tmp' with cruft, but I hope this doesn't cause
too much in the way of diagnostic trouble.