cleanup: Map test files according to structure in src

For example cmd_account() is located in src/command/cmd_funcs.c.
The unit test was located in tests/unittests/test_cmd_account.c.
Let's move it to a subdirectory like tests/unittests/command/test_cmd_account.c
to correpsond to the same structure.

tests/unittests/xmpp/test_chat_session.h

@@ -0,0 +1,4 @@
+void returns_false_when_chat_session_does_not_exist(void** state);
+void creates_chat_session_on_recipient_activity(void** state);
+void replaces_chat_session_on_recipient_activity_with_different_resource(void** state);
+void removes_chat_session(void** state);