tests: fix portability issue in new dd/ascii test

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.