fate: fix md5sum replacement on some systems

On systems where the 'md5' command is used, there is a conflict
with the md5() shell function in fate-run.sh.  Using the 'command'
keyword bypasses the shell function for correct behaviour.
Signed-off-by: Mans Rullgard <mans@mansr.com>

tests/md5.sh

@@ -2,8 +2,8 @@
 
 if [ X"$(echo | md5sum 2> /dev/null)" != X ]; then
     do_md5sum() { md5sum -b $1; }
-elif [ X"$(echo | md5 2> /dev/null)" != X ]; then
-    do_md5sum() { md5 $1 | sed 's#MD5 (\(.*\)) = \(.*\)#\2 *\1#'; }
+elif [ X"$(echo | command md5 2> /dev/null)" != X ]; then
+    do_md5sum() { command md5 $1 | sed 's#MD5 (\(.*\)) = \(.*\)#\2 *\1#'; }
 elif [ -x /sbin/md5 ]; then
     do_md5sum() { /sbin/md5 -r $1 | sed 's# \**\./# *./#'; }
 elif openssl version >/dev/null 2>&1; then