t4053: avoid race when killing background processes

The test 'diff --no-index reads from pipes' starts a couple of
background processes that write to the pipes that are passed to "diff
--no-index". If the test passes then we expect these processes to exit
as all their output will have been read. However if the test fails
then we want to make sure they do not hang about on the users machine
and the test remembers they should be killed by calling

      test_when_finished  "! kill $!"

after each background process is created. Unfortunately there is a
race where test_when_finished may run before the background process
exits even when all its output has been read resulting in the kill
command succeeding which causes the test to fail. Fix this by ignoring
the exit status of the kill command. If the diff is successful we
could instead wait for the background process to exit and check their
status but that feels like it is testing the platform's printf
implementation rather than git's code.

Reported-by: Jeff King <peff@peff.net>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4053-diff-no-index.sh

@@ -248,11 +248,11 @@ test_expect_success PIPE,SYMLINKS 'diff --no-index reads from pipes' '
 	{
 		(test_write_lines a b c >old) &
 	} &&
-	test_when_finished "! kill $!" &&
+	test_when_finished "kill $! || :" &&
 	{
 		(test_write_lines a x c >new) &
 	} &&
-	test_when_finished "! kill $!" &&
+	test_when_finished "kill $! || :" &&
 
 	cat >expect <<-EOF &&
 	diff --git a/old b/new-link