unit-tests: Add mock sender_t implementation for unit testing

This allows to retrieve packets sent by an IKE_SA and pass it to another
IKE_SA directly via process_message().

src/libcharon/tests/libcharon_tests.h

@@ -1,4 +1,7 @@
 /*
+ * Copyright (C) 2014-2016 Tobias Brunner
+ * HSR Hochschule fuer Technik Rapperswil
+ *
  * Copyright (C) 2014 Martin Willi
  * Copyright (C) 2014 revosec AG
  *
@@ -13,6 +16,14 @@
  * for more details.
  */
 
+/**
+ * @defgroup libcharon-tests tests
+ * @ingroup libcharon
+ *
+ * @defgroup test_utils_c test_utils
+ * @ingroup libcharon-tests
+ */
+
 TEST_SUITE(ike_cfg_suite_create)
 TEST_SUITE(mem_pool_suite_create)
 TEST_SUITE_DEPEND(message_chapoly_suite_create, AEAD, ENCR_CHACHA20_POLY1305, 32)