Fix a TLSProxy race condition

TLSProxy starts s_server and specifies the number of client connects
it should expect. After that s_server is supposed to close down
automatically. However, if another test is then run then TLSProxy
will start a new instance of s_server. If the previous instance
hasn't closed down yet then the new instance can fail to bind to
the socket.

Reviewed-by: Richard Levitte <levitte@openssl.org>