selftests/net: packetdrill: make tcp buf limited timing tests benign

selftests/net: packetdrill: make tcp buf limited timing tests benign

The following tests are failing on debug kernels:

  tcp_tcp_info_tcp-info-rwnd-limited.pkt
  tcp_tcp_info_tcp-info-sndbuf-limited.pkt

with reports like:

      assert 19000 <= tcpi_sndbuf_limited <= 21000, tcpi_sndbuf_limited; \
  AssertionError: 18000

and:

      assert 348000 <= tcpi_busy_time <= 360000, tcpi_busy_time
  AssertionError: 362000

Extend commit 912d6f669725 ("selftests/net: packetdrill: report benign
debug flakes as xfail") to cover them.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250115232129.845884-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>