test/ruby/test_process.rb: avoid killing wrong parent

We must capture intended target PID before forking, since
Process.ppid may change if parent dies (e.g. due to timeout)

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47968 b2dd03c8-39d4-4d8f-98ff-823fe69b080e