vty_test_runner.py: fix socket leak

Each running test would open up another socket without ever closing unused
ones. Close the sockets after each test is done.

Change-Id: Ie433c8560de54f9a9d05fa07c44bae3126d19b30
diff --git a/openbsc/tests/vty_test_runner.py b/openbsc/tests/vty_test_runner.py
index f434360..8b17397 100644
--- a/openbsc/tests/vty_test_runner.py
+++ b/openbsc/tests/vty_test_runner.py
@@ -57,6 +57,8 @@
         self.vty = obscvty.VTYInteract(appstring, "127.0.0.1", appport)
 
     def tearDown(self):
+        if self.vty:
+            self.vty._close_socket()
         self.vty = None
         osmoutil.end_proc(self.proc)