test_gc.rb: workaround for Windows

* test/ruby/test_gc.rb (test_interrupt_in_finalizer): use inner
  process signal for Windows.  [ruby-core:66825] [Bug #10595]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48833 b2dd03c8-39d4-4d8f-98ff-823fe69b080e