How can you actually force garbage collection to run fully for unit testing?

output.txt

will define finalizer
will run gc
did run gc
finalize was called

try-to-force-garbage-collection.rb

# I'm trying write some unit tests around some finalization logic.
# What I would like is some way to make sure that a given object was garbage
# collected. The only surefire way that I  know to do this is to assert
# that a finalizer was called.
#
# The problem is knowing when all finalizers that can be run, have been run, so
# how to accomplish this?
$stderr.puts "will define finalizer"
ObjectSpace.define_finalizer(Object.new, proc { $stderr.puts "finalize was called" })

$stderr.puts "will run gc"
GC.start full_mark: true, immediate_sweep: true

# I think the GC has collected, but the finalizers are queued somewhere to run
# How can we make them run?!?
Thread.pass

# I'd like to put the assertion here.
$stderr.puts "did run gc"

How about this?

class TheFinalOne
  def initialize(thing)
    ObjectSpace.define_finalizer(self, self.class.finalize(thing))
  end

  def self.finalize(thing)
    proc { puts thing }
  end
end

# spec
describe TheFinalOne do
  it "a winner is you" do
    expect(TheFinalOne).to receive(:finalize).with("some thing")
    TheFinalOne.new("some thing")
  end
end

Sadly, I end up with the same result. I'm beginning to think that it will require a C extension.