[test suite] apply loopback interface to invalid pcap filter test

test/test-fwknop.pl

@@ -185,7 +185,7 @@ my $test_limit = 0;
 my $list_mode = 0;
 my $diff_dir1 = '';
 my $diff_dir2 = '';
-my $loopback_intf = '';
+our $loopback_intf = '';
 my $default_pkt_tries = 10;
 my $send_all_loop_once = 0;
 my $detect_server_loop_once = 0;

test/tests/basic_operations.pl

@@ -2654,7 +2654,8 @@
         'subcategory' => 'server',
         'detail'   => 'invalid pcap filter',
         'function' => \&generic_exec,
-        'cmdline' => "$fwknopdCmd $default_server_conf_args -f -P proto invalid",
+        'cmdline' => "$fwknopdCmd $default_server_conf_args " .
+            "-i $loopback_intf -f -P proto invalid",
         'exec_err' => $YES,
     },
     {