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/command/test_cmd_join.h

@@ -0,0 +1,9 @@
+void cmd_join_shows_message_when_disconnecting(void** state);
+void cmd_join_shows_message_when_connecting(void** state);
+void cmd_join_shows_message_when_disconnected(void** state);
+void cmd_join_shows_message_when_undefined(void** state);
+void cmd_join_shows_error_message_when_invalid_room_jid(void** state);
+void cmd_join_uses_account_mucservice_when_no_service_specified(void** state);
+void cmd_join_uses_supplied_nick(void** state);
+void cmd_join_uses_account_nick_when_not_supplied(void** state);
+void cmd_join_uses_password_when_supplied(void** state);