selftests: mptcp: join: colored results

Thanks to the parent commit, it is easy to change the output and add
some colours to help spotting issues.

The colours are not used if stdout is redirected or if NO_COLOR env var
is set to 1 as specified in https://no-color.org

.

It is possible to force displaying the colours even if stdout is
redirected by setting this env var:

  SELFTESTS_MPTCP_LIB_COLOR_FORCE=1

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-v1-2-7e9cc530a9cd@tessares.net


Signed-off-by: Jakub Kicinski <kuba@kernel.org>