Revert "test/socket/test_socket.rb: skip on Solaris"

This reverts commit 27fb9d272d.

The test failure on Solaris 10 is due to incomplete IPv6 configuration
on the CI server, that have already been fixed.

Reference for the fix: https://centrify.force.com/support/Article/KB-1179-X11-Forwarding-fails-with-Centrify-OpenSSH-5-0-Solaris/

test/socket/test_socket.rb

@@ -1,9 +1,5 @@
 # frozen_string_literal: true
 
-# tentatively disabled due to IPv6 configuration issue on Solaris CI
-# http://rubyci.s3.amazonaws.com/solaris10-gcc/ruby-master/log/20210729T040002Z.fail.html.gz
-return if /solaris/ =~ RUBY_PLATFORM
-
 begin
   require "socket"
   require "tmpdir"