accept_nonblock_spec.rb: skip spurious failure

on solaris until it gets stable.
https://rubyci.org/logs/rubyci.s3.amazonaws.com/unstable11x/ruby-trunk/log/20181227T142405Z.fail.html.gz

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66593 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/ruby/library/socket/tcpserver/accept_nonblock_spec.rb

@@ -63,7 +63,7 @@ describe 'TCPServer#accept_nonblock' do
       end
     end
 
-    platform_is_not :windows do # spurious
+    platform_is_not :windows, :solaris do # spurious
       describe 'with a connected client' do
         before do
           @client = TCPSocket.new(ip_address, @server.connect_address.ip_port)