tests: Solaris cmp complains about empty files, even with -s

When you compare an empty file, such as /dev/null, with a non-empty file,
Solaris cmp complains on stderr with "cmp: EOF on /dev/null", even if the
-s argument is present.  GNU cmp makes the complaint, but silences it with
-s.  We can change the pdiff utility to simply redirect stderr to /dev/null
so that we don't have to worry about this difference in the test files.

tests/pdiff

@@ -25,7 +25,7 b' filediff(){'
         file2=/dev/null
     fi
 
-    if cmp -s "$file1" "$file2"; then
+    if cmp -s "$file1" "$file2" 2> /dev/null; then
         # Return immediately, because comparison isn't needed. This
         # also avoids redundant message of diff like "No differences
         # encountered" (on Solaris)