为什么在Minitest测试中fork以非零退出?

代码如下:

require 'minitest/autorun'
class Foo < Minitest::Test
  def test_foo
    Process.fork do
      exit(0)
    end
    p Process.waitall
  end
end

这是输出:

$ ruby a.rb
Run options: --seed 40445

# Running:

[[41827, #<Process::Status: pid 41827 exit 1>]]
.

Finished in 0.016218s, 61.6599 runs/s, 0.0000 assertions/s.
1 runs, 0 assertions, 0 failures, 0 errors, 0 skips

为什么退出代码不是零?

没有Minitest的相同代码可以正常工作:

Process.fork do
  exit(0)
end
p Process.waitall

发生了什么?