tools: ynl: convert rt-addr sample to selftest

tools: ynl: convert rt-addr sample to selftest

Convert rt-addr.c to use kselftest_harness.h with FIXTURE/TEST_F.

Validate that the addresses configured by the wrapper (192.168.1.1
and 2001:db8::1) appear in the dump.

Output:

  TAP version 13
  1..1
  # Starting 1 tests from 1 test cases.
  #  RUN           rt_addr.dump ...
  #               lo: 127.0.0.1
  #            nsim0: 192.168.1.1
  #               lo: ::1
  #            nsim0: 2001:db8::1
  #            nsim0: fe80::7c66:c9ff:fe5f:bf01
  #            OK  rt_addr.dump
  ok 1 rt_addr.dump
  # PASSED: 1 / 1 tests passed.
  # Totals: pass:1 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/20260307033630.1396085-10-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>