helpers/labgrid-raw-interface: fix tcpdump timeout handling

The current tcpdump timeout handling does not work.

This is supposed to exit after 5 seconds:

  tcpdump --interface=eth0 -w - -G 5 -W 1

But that does not work if no packets are received.

So in order to make the timeout actually work, prefix tcpdump with
coreutils' timeout command instead:

  timeout --signal=INT --preserve-status <timeout> <tcpdump command>

By sending SIGINT (`--signal=INT`), tcpdump exits gracefully and finishes
writing received packets. Combined with `--preserve-status`, tcpdump's
actual exit status is preserved, being 0 even on SIGINT if nothing else
went wrong.

An alternative would be to use subprocess, but that would prevent us
from using `os.execvp()`, replacing the process. So the timeout command
is the better approach here.

Signed-off-by: Bastian Krause <bst@pengutronix.de>

Author: Bastian-Krause

helpers/labgrid-raw-interface

@@ -73,12 +73,7 @@ def main(program, options):
             args.append(str(options.count))
 
         if options.timeout:
-            # The timeout is implemented by specifying the number of seconds before rotating the
-            # dump file, but limiting the number of files to 1
-            args.append("-G")
-            args.append(str(options.timeout))
-            args.append("-W")
-            args.append("1")
+            args = ["timeout", "--signal=INT", "--preserve-status", str(options.timeout)] + args
 
     elif program == "ip":
         args.append("link")