tools: ynl: convert rt-link sample to selftest

tools: ynl: convert rt-link sample to selftest

Convert rt-link.c to use kselftest_harness.h with FIXTURE/TEST_F.
Move rt-link from BINS to TEST_GEN_PROGS.

Output:

  TAP version 13
  1..3
  # Starting 3 tests from 1 test cases.
  #  RUN           rt_link.dump ...
  #   1:          lo: mtu 65536
  #   2:          sit0: mtu  1480  kind sit
  #            OK  rt_link.dump
  ok 1 rt_link.dump
  #  RUN           rt_link.netkit ...
  #   4:          nk1: mtu  1500  kind netkit    primary 1  policy blackhole
  #            OK  rt_link.netkit
  ok 2 rt_link.netkit
  #  RUN           rt_link.netkit_err_msg ...
  #            OK  rt_link.netkit_err_msg
  ok 3 rt_link.netkit_err_msg
  # PASSED: 3 / 3 tests passed.
  # Totals: pass:3 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/20260307033630.1396085-5-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>