test: skip pre-dump images in socket-tcp*.hook

These hooks opens inetsk.img, but it doesn't exist if
processes were pre-dumped.

travis-ci: success for test: skip pre-dump images in socket-tcp*.hook
Reported-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

test/zdtm/static/socket-tcp-fin-wait1.hook

@@ -22,7 +22,11 @@ if sys.argv[1] != "--post-restore":
 print "Check TCP images"
 
 def get_sockets(image_dir):
-    f = open(os.path.join(image_dir, "inetsk.img"))
+    fname = os.path.join(image_dir, "inetsk.img")
+    if not os.access(fname, os.F_OK):
+        return None
+
+    f = open(fname)
     sockets = pycriu.images.load(f)
     sockets = sockets["entries"]