test_process.rb: WIFEXITED and so on are system dependent

* test/ruby/test_process.rb (test_status_kill): WIFEXITED and so
  on are available only if signal is supported by the system.
  [ruby-dev:48203] [Bug #9820]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45918 b2dd03c8-39d4-4d8f-98ff-823fe69b080e