remove dependency on xxd

Fixes #3797

tests/printf.in

@@ -28,11 +28,11 @@ printf 'a\cb'
 echo
 
 # Bogus printf specifier, should produce no stdout
-printf "%5" 10 ^ /dev/null
+printf "%5" 10 ^/dev/null
 
 # Octal escapes produce literal bytes, not characters
 # \376 is 0xFE
-printf '\376' | xxd -p
+printf '\376' | display_bytes
 
 # Verify that floating point conversions and output work correctly with
 # different combinations of locales and floating point strings. See issue