tools: ynl: convert devlink sample to selftest

tools: ynl: convert devlink sample to selftest

Convert devlink.c to use kselftest_harness.h with FIXTURE/TEST_F.
Move devlink from BINS to TEST_GEN_FILES in the Makefile since
it's invoked via the devlink.sh wrapper which sets up netdevsim.

Output:

  TAP version 13
  1..2
  # Starting 2 tests from 1 test cases.
  #  RUN           devlink.dump ...
  # netdevsim/netdevsim1337
  #            OK  devlink.dump
  ok 1 devlink.dump
  #  RUN           devlink.info ...
  # netdevsim/netdevsim1337:
  #   driver: netdevsim
  #   running fw:
  #     fw.mgmt: 10.20.30
  #            OK  devlink.info
  ok 2 devlink.info
  # PASSED: 2 / 2 tests passed.
  # Totals: pass:2 fail:0 xfail:0 xpass:0 skip:0 error:0

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Tested-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20260307033630.1396085-8-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>