Keep test daemon sockets under the path limit

Keep test daemon sockets under the path limit

The test daemon keeps its data, and so its socket, in a short directory
under /tmp. t.TempDir holds the test name and the long TMPDIR of the
development shell, so a test with a long name failed to listen on a
socket path over 107 bytes, and bin/check failed in the shell unless
TMPDIR was set short by hand.