selftests: net-drv: exercise queue stats when the device is down

Verify that total device stats don't decrease after it has been turned down.
Also make sure the device doesn't crash when we access per-queue stats
when it's down (in case it tries to access some pointers that are NULL).

  KTAP version 1
  1..5
  ok 1 stats.check_pause
  ok 2 stats.check_fec
  ok 3 stats.pkt_byte_sum
  ok 4 stats.qstat_by_ifindex
  ok 5 stats.check_down
  # Totals: pass:5 fail:0 xfail:0 xpass:0 skip:0 error:0

v3:
- use errno.EOPNOTSUPP (Petr)
- move qstat[0] under try (Petr)

v2:
- KTAP output formatting (Jakub)
- defer instead of try/finally (Jakub)
- disappearing stats is an error (Jakub)
- ksft_ge instead of open coding (Jakub)

Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20240802000309.2368-1-sdf@fomichev.me


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