Handle printing to closed pipes better in tests.

Some systems do not deliver SIGPIPE to shell pipeline writers when the
downstream reader has exited.  So, also check for errors from
/bin/printf.  I fixed this for OpenBSD 6.0 but I think it will also
fix posh on Linux.

src/tests/prediction-unicode.test

@@ -52,8 +52,8 @@ tmux_commands()
     sleep 1
     printf "send-keys 0x04\n"
     # This will get killed by SIGPIPE.
-    while sleep 1; do
-	printf "show-options\n"
+    while printf "show-options\n"; do
+	sleep 1
     done
 }