Patch to allow us to specify an alternate tcpdump binary when running tests.

This patch should be sent upstream.

--- tcpdump-4.7.4/tests/TESTonce.orig	2016-11-22 07:54:46.080609871 +0000
+++ tcpdump-4.7.4/tests/TESTonce	2016-11-22 09:51:15.439509866 +0000
@@ -1,5 +1,7 @@
 #!/usr/bin/env perl
 
+$TCPDUMP = "../tcpdump" if (!($TCPDUMP = $ENV{TCPDUMP_BIN}));
+
 system("mkdir -p NEW DIFF");
 
 if(@ARGV != 4) {
@@ -21,7 +23,7 @@
 else {
     # we used to do this as a nice pipeline, but the problem is that $r fails to
     # to be set properly if the tcpdump core dumps.
-    $r = system "../tcpdump 2>/dev/null -n -r $input $options >NEW/$output";
+    $r = system "$TCPDUMP 2>/dev/null -n -r $input $options >NEW/$output";
     if($r == 0x100) {
         # this means tcpdump exited with code 1.
         open(OUTPUT, ">>"."NEW/$output") || die "fail to open $output\n";