selftests: mptcp: userspace_pm: uniform results printing

There are a few reasons to do that:

- When the tabs are not printed as 8 spaces, some results were not
  properly aligned

- Some lines printing the test name were very long due to the use of a
  lot of spaces/tabs at the end and stdbuf at the beginning.

- To reduce duplicated code, e.g. to print what has failed and set the
  status

But by centralising how the test results are printed, this also prepares
future commits to avoid more duplicated code and ease the tracking of
the different subtests.
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>