selftests: mptcp: join: rework detailed report

This patch modifies how the detailed results are printed, mainly to
improve what is displayed in case of issue:

- Now the test name (title) is printed earlier, when starting the test
  if it is not intentionally skipped: by doing that, errors linked to
  a test will be printed after having written the test name and then
  avoid confusions.

- Due to the previous item, it is required to add a new line after
  having printed the test name because in case of error with a command,
  it is better not to have the output in the middle of the screen.

- Each check is printed on a dedicated line with aligned status (ok,
  skip, fail): it is easier to spot which one has failed, simpler to
  manage in the code not having to deal with alignment case by case and
  helpers can be used to uniform what is done. These helpers can also be
  useful later to do more actions depending on the results or change in
  one place what is printed.

- Info messages have been reduced and aligned as well. And info messages
  about the creation of the default test files of 1 KB are no longer
  printed.

Example:

  001 no JOIN
        syn                                 [ ok ]
        synack                              [ ok ]
        ack                                 [ ok ]

Or with a skip and a failure:

  001 no JOIN
        syn                                 [ ok ]
        synack                              [fail] got 42 JOIN[s] synack expected 0
  Server ns stats
  (...)
  Client ns stats
  (...)
        ack                                 [skip]

Or with info:

  104 Infinite map
        Test file (size 128 KB) for client
        Test file (size 128 KB) for server
        file received by server has inverted byte at 169
        5 corrupted pkts
        syn                                 [ ok ]
        synack                              [ ok ]

While at it, verify_listener_events() now also print more info in case
of failure and in pm_nl_check_endpoint(), the test is marked as failed
instead of skipped if no ID has been given (internal selftest issue).

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Link: https://lore.kernel.org/r/20230730-upstream-net-next-20230728-mptcp-selftests-misc-v1-1-7e9cc530a9cd@tessares.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>