Add mbedtls_mock_socket to SSL unit tests

In a unit test we want to avoid accessing the network. To test the
handshake in the unit test suite we need to implement a connection
between the server and the client. This socket implementation uses
two ring buffers to mock the transport layer.
diff --git a/tests/suites/test_suite_ssl.data b/tests/suites/test_suite_ssl.data
index 8e43852..5b670f9 100644
--- a/tests/suites/test_suite_ssl.data
+++ b/tests/suites/test_suite_ssl.data
@@ -31,6 +31,9 @@
 Callback buffer test: Reading from empty buffer
 test_callback_buffer:50:0:0:10:0:0:0:0:0
 
+Test mock TCP connection
+ssl_mock_tcp:
+
 SSL DTLS replay: initial state, seqnum 0
 ssl_dtls_replay:"":"000000000000":0