t6026-merge-attr: don't fail if sleep exits early

Commit 5babb5b ("t6026-merge-attr: clean up background process at end
of test case") added a kill command to clean up after the test, but this
can fail if the sleep command exits before the cleanup is executed.
Ignore the error from the kill command.

Signed-off-by: Andreas Schwab <[email protected]>
Signed-off-by: Jeff King <[email protected]>

Author: andreas-schwab

t/t6026-merge-attr.sh

@@ -187,7 +187,7 @@ test_expect_success 'custom merge does not lock index' '
 		sleep 1 &
 		echo $! >sleep.pid
 	EOF
-	test_when_finished "kill \$(cat sleep.pid)" &&
+	test_when_finished "kill \$(cat sleep.pid) || :" &&
 
 	test_write_lines >.gitattributes \
 		"* merge=ours" "text merge=sleep-one-second" &&