[ruby/ipaddr] Remove warning by asserting its presence

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	/tmp/test/test_ipaddr.rb:202: warning: IPAddr#ipv4_compat is obsolete

ruby/ipaddr@31d62407c2

Author: etiennebarrie

test/test_ipaddr.rb

@@ -199,7 +199,9 @@ def test_ipv4_compat
     assert_equal(128, b.prefix)
 
     a = IPAddr.new("192.168.0.0/16")
-    b = a.ipv4_compat
+    assert_warning(/obsolete/) {
+      b = a.ipv4_compat
+    }
     assert_equal("::192.168.0.0", b.to_s)
     assert_equal(Socket::AF_INET6, b.family)
     assert_equal(112, b.prefix)