Remove warning by asserting its presence

[etiennebarrie]

$ rake test >/dev/null
/tmp/test/test_ipaddr.rb:202: warning: IPAddr#ipv4_compat is obsolete

This shows up in Ruby's CI too.

[taketo1113]

Thanks for fixing the warning.
It is related to the Pull Request, and I forgot to include the assert_warning.

• Fix #ipv4_compat returning invalid prefix #86

It confirms that the following warning appears on RubyCI:

[ 5514/28960] TC_IPAddr#test_ipv4_compat/home/chkbuild/chkbuild/tmp/build/20250930T183003Z/ruby/test/test_ipaddr.rb:202: warning: IPAddr#ipv4_compat is obsolete
 = 0.00 s

https://rubyci.s3.amazonaws.com/debian11/ruby-master/log/20250930T183003Z.log.html.gz

As the ipaddr gem’s CI does not show any warnings, I will add the CI execution options to make the warnings appear.

(Updated) The warning also appears in the ipaddr gem's CI.

Run bundle exec rake test
  bundle exec rake test
  shell: /usr/bin/bash -e {0}
/home/runner/work/ipaddr/ipaddr/test/test_ipaddr.rb:202: warning: IPAddr#ipv4_compat is obsolete
Finished in 0.019228222 seconds.
32 tests, 472 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications

https://github.com/ruby/ipaddr/actions/runs/18051618316/job/51374448742

[hsbt]

👍 Thanks both!