t1400: have fifo test clean after itself

One test in this script creates a pair of FIFOs, "in" and "out", that are named so generically that later tests may be tempted to use them. By the time those later tests run a command with its output redirected to the file (e.g., "git foobar >out"), however, nobody is reading from the lingering FIFO, and the test gets blocked forever. Clean them up when the test finishes. Signed-off-by: Junio C Hamano <gitster@pobox.com>