selftests: mptcp: diag: format subtests results in TAP

The current selftests infrastructure formats the results in TAP 13. This
version doesn't support subtests and only the end result of each
selftest is taken into account. It means that a single issue in a
subtest of a selftest containing multiple subtests forces the whole
selftest to be marked as failed. It also means that subtests results are
not tracked by CIs executing selftests.

MPTCP selftests run hundreds of various subtests. It is then important
to track each of them and not one result per selftest.

It is particularly interesting to do that when validating stable kernels
with the last version of the test suite: tests might fail because a
feature is not supported but the test didn't skip that part. In this
case, if subtests are not tracked, the whole selftest will be marked as
failed making the other subtests useless because their results are
ignored.

This patch formats subtests results in TAP in diag.sh selftest.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
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>