Update TESTonce

Adapt script for Strawberry Perl and possibly other Win32 Perls. The program is windump.exe on Windows.
diff --git a/tests/TESTonce b/tests/TESTonce
index 2397793..6eddbd2 100755
--- a/tests/TESTonce
+++ b/tests/TESTonce
@@ -12,7 +12,12 @@
 $output=$ARGV[2];
 $options=$ARGV[3];
 
-$r = system "../tcpdump 2>/dev/null -n -r $input $options | tee NEW/$output | diff -w $output - >DIFF/$output.diff";
+if ($^O eq 'MSWin32') {
+  $r = system "..\\windump -n -r $input $options 2>NUL | tee NEW/$output | diff -w $output - >DIFF/$output.diff";
+ }
+else {
+  $r = system "../tcpdump 2>/dev/null -n -r $input $options | tee NEW/$output | diff -w $output - >DIFF/$output.diff";
+}
 
 if($r == 0) {
   printf "    %-30s: passed\n", $name;