How is Cabal's Distribution.TestSuite.Progress datatype supposed to be used?

I have a simple TestInstance in a detailed-0.9 test suite:

tests :: IO [Test]
tests = return [Test testInstance]

testInstance :: TestInstance
testInstance =
  TestInstance
    { run = ioProgress,
      name = "test 1",
      tags = [],
      options = [],
      setOption = \_ _ -> Left ""
    }

ioProgress :: IO Progress
ioProgress = progress 3
  where
    progress :: Int -> IO Progress
    progress 0 = return (Finished (Fail "failed"))
    progress n = return (Progress ("n == " ++ show n) (progress (n - 1)))

I thought that maybe I would see some output like

n == 3
n == 2
n == 1

whilst the test runs but I never see these strings, even with --test-show-details=always and --test-show-details=streaming.

The String in the Progress constructor doesn't appear to be used for anything. (I was able to build the Cabal package from source after replacing it with an Int, for example.)

The code that implements testing for detailed-0.9 is in Distribution.Simple.Test.LibV09, and it just keeps unwrapping Progress constructors (ignoring the String field) until it finds a Finished:

stubRunTests tests = do
  ...
      where
        finish (Finished result) =
          return
            TestLog
              { testName = name t
              , testOptionsReturned = defaultOptions t
              , testResult = result
             }
        finish (Progress _ next) = next >>= finish
  ...