fate: improve md5sum utility selection

The 'md5sum' command is used with the -b flag so the presence test
must also use this flag.
Signed-off-by: Mans Rullgard <mans@mansr.com>

tests/md5.sh

 # try to find an md5 program
 
-if [ X"$(echo | md5sum 2> /dev/null)" != X ]; then
+if [ X"$(echo | md5sum -b 2> /dev/null)" != X ]; then
     do_md5sum() { md5sum -b $1; }
 elif [ X"$(echo | command md5 2> /dev/null)" != X ]; then
     do_md5sum() { command md5 $1 | sed 's#MD5 (\(.*\)) = \(.*\)#\2 *\1#'; }