selftests: drv-net: convert so_txtime to drv-net

selftests: drv-net: convert so_txtime to drv-net

In preparation for extending to pacing hardware offload, convert the
so_txtime.sh test to a drv-net test that can be run against netdevsim
and real hardware.

Also update so_txtime.c to not exit on first failure, but run to
completion and report exit code there. This helps with debugging
unexpected results, especially when processing multiple packets,
as happens in the "reverse_order" testcase.

Signed-off-by: Willem de Bruijn <willemb@google.com>
----

v6 -> v7

- update test to use new argument expect_fail
- v6 received Reviewed-by, but dropped due to above (minor) change

v5 -> v6

- fix order in tools/testing/selftests/drivers/net/config

v4 -> v5

- move qdisc setup/restore into each test
- add tc to utils.py (separate patch)
- test expected failure (separate patch)
- fix pylint
- convert fail to pass for timing errors if KSFT_MACHINE_SLOW
  (cmd does not special case KSFT_SKIP process returncode yet)

Responses to sashiko review

- The test converts per packet failure to errors, to continue
  testing other packets, but other error() cases are not in scope.
- The test starts sender and receiver at an absolute future time,
  like the original test. This assumes ~msec scale sync'ed clocks.
- The tc qdisc replace command works fine with noqueue. Tested
  manually.

v3 -> v4

- restore original qdisc after test
- drop unnecessary underscore in tap test names

v2 -> v3

- Makefile: so_txtime from YNL_GEN_FILES to TEST_GEN_FILES (Sashiko, NIPA)

v1 -> v2
- move so_txtime.c for net/lib to drivers/net (Jakub)
- fix drivers/net/config order (Jakub)
- detect passing when failure is expected (Jakub, Sashiko)
- pass pylint --disable=R (Jakub)
- only call ksft_run once (Jakub)
- do not sleep if waiting time is negative (Sashiko)
- add \n when converting error() to fprintf() (Sashiko)
- 4 space indentation, instead of 2 space
- increase sync delay from 100 to 200ms, to fix rare vng flakes

Link: https://patch.msgid.link/20260504174056.565319-4-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>