t1400: have fifo test clean after itself

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>