Better testing for finalization

从本 issue 中提出的 finalizer 规范开始，并审查 jruby/jruby#7267 中描述的行为。确定 suite 如何执行由 GC 触发的 finalization 并隔离警告输出；完成标准是覆盖最终发生的 finalization，以及一个 finalizer 中的异常不会影响下一个 finalizer 的情况。

While attempting to write a spec for jruby/jruby#7267 I ran into various issues and questions...

1. There are no specs testing that GC eventually finalizes objects. This is obviously difficult to predict, but it is behavior I believe we should be testing one way or another.
2. I was testing that exceptions in one finalizer are not seen by the next finalizer, but could not figure out a way to eliminate the warning output from Ruby indicating that a finalizer raised an exception.

The spec I attempted is below, but only makes a best attempt at forcing GC-oriented finalization and still does not suppress the error output.

  it "hides raised exceptions from one finalizer to the next" do
    def scoped(result)
        Proc.new { result << "ok" if $!.nil?; raise }
    end
    def test(result)
      obj = "Test"
      # finalizer order may vary so both handlers check $! and raise an error
      ObjectSpace.define_finalizer(obj, scoped(result))
      ObjectSpace.define_finalizer(obj, scoped(result))
    end

    result = []
    begin
      old_verbose, $VERBOSE = $VERBOSE, false
      test(result)
    ensure
      $VERBOSE = old_verbose
    end

    100.times { GC.start; break if result.size == 2 }

    result.should == ["ok", "ok"]
  end

I don't want to leave the fix for jruby/jruby#7267 untested, but I'm unsure how we should move forward to improve the GC-triggered finalization specs.