test: reconnect to console after server restart

The scenario is the following:

1. Create and start a server (TarantoolServer instance).
2. Stop it.
3. Call `<server>.admin(<command>)`.

I intend to follow this scenario in a test case in the following commit.

We should close old socket at stopping the server, because an attempt to
write to the old socket may not fail even if the other end is already
gone. If we'll close the console connection (TarantoolAdmin instance)
explicitly, it'll be reconnected at the next usage.

Part of #111

Author: Totktonada

unit/suites/lib/tarantool_server.py

@@ -194,6 +194,7 @@ def start(self):
         self.wait_until_started()
 
     def stop(self):
+        self.admin.disconnect()
         if self.process.poll() is None:
             self.process.terminate()
             self.process.wait()