tests: cover more Diag-related code

[matttbe]

There are some improvements that can be done on the tests side to cover more code in some areas (code coverage), e.g. the Diag part:

• subflow_get_info_size() from diag.c is not covered
• mptcp_diag_dump_one() from mptcp_diag.c is not covered

[geliangtang]

Hi Matt, I assigned this issue to myself since my colleague Gang Yan in my group is looking at it.

[matttbe]

Thanks! A good task to get familiar with the code and the test suite.

Please note that subflow_get_info_size() might move out of net/mptcp with this patch, but if it is easy, it still makes sense to have a test covering this part.