selftests: mptcp: join: no SKIP mark for group checks

selftests: mptcp: join: no SKIP mark for group checks

When executing the last MPTCP selftests on older kernels, this output is
printed:

  # 001 no JOIN
  #       join Rx                             [SKIP]
  #       join Tx                             [SKIP]
  #       fallback                            [SKIP]

In fact, behind each line, a few counters are checked, and likely not
all of them have been skipped because the they are not available on
these kernels. Instead, "new" and unsupported counters for these groups
are now ignored, and [ OK ] will be printed instead of [SKIP].

Note that on the MPTCP CI, when validating the dev versions, any
unsupported counter will cause the tests to fail. So this is safe not to
print 'SKIP' for these group checks.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260203-net-next-mptcp-misc-feat-6-20-v1-15-31ec8bfc56d1@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>