test_process.rb: deadlock test

* test/ruby/test_process.rb (test_deadlock_by_signal_at_forking): test
  for r44687, deadlock in rb_fork_internal().

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44689 b2dd03c8-39d4-4d8f-98ff-823fe69b080e