tests: fix portability issue in new dd/ascii test

* tests/dd/ascii.sh: Quote so that '\\' is passed
to printf rather than a single '\', as that's not portable.
bash, dash, zsh and external solaris printf were seen to support
  printf '\%03o' 1 2
while ksh, freebsh shell and external GNU printf need the more portable
  printf '\\%03o' 1 2
Note we don't use env here to call the coreutils printf implementation,
as there are many printf calls, so relying on portable shell
implementations will be faster.

tests/dd/ascii.sh

@@ -22,7 +22,7 @@ print_ver_ dd
 {
   # Two lines, EBCDIC " A A" and " A  ", followed by all the bytes in order.
   printf '\100\301\100\301\100\301\100\100' &&
-  printf $(printf "\\%03o" $(seq 0 255 ));
+  printf $(printf '\\%03o' $(seq 0 255));
 } >in || framework_failure_
 
 {