Fix transient failures in Signal.trap specs
Fix transient failures in Signal.trap specs

* We cannot `Process.kill` after `Signal.trap(sig, :IGNORE)` as we have
  no way to wait for the signal to be received and know about it.
  Therefore the signal could be delayed until after the default handler is
  restored and the process would exit with exit code 129 (killed by SIGHUP).

ruby/spec,nobu/rubyspec,ruby/spec,ruby/spec,nobu/rubyspec,nobu/rubyspec