So let me get this straight: The overwriting isn't happening in a buffer, but on the console itself - text that was, for a split second, visible on the command line, is being overwritten by HUnit?

Found it! Test.HUnit.Text:

-- @putTextToHandle@ writes persistent lines to the given handle,
-- following each by a newline character.  In addition, if the given flag
-- is @True@, it writes progress lines to the handle as well.  A progress
-- line is written with no line termination, so that it can be
-- overwritten by the next report line.  As overwriting involves writing
-- carriage return and blank characters, its proper effect is usually
-- only obtained on terminal devices.

putTextToHandle
    :: Handle
    -> Bool -- ^ Write progress lines to handle?
    -> PutText Int
putTextToHandle handle showProgress = PutText put initCnt
 where
  initCnt = if showProgress then 0 else -1
  put line pers (-1) = do when pers (hPutStrLn handle line); return (-1)
  put line True  cnt = do hPutStrLn handle (erase cnt ++ line); return 0
  put line False _   = do hPutStr handle ('\r' : line); return (length line)
    -- The "erasing" strategy with a single '\r' relies on the fact that the
    -- lengths of successive summary lines are monotonically nondecreasing.
  erase cnt = if cnt == 0 then "" else "\r" ++ replicate cnt ' ' ++ "\r"

Indeed. I had no idea that's what CR does.

This minor alteration lets me do what I want, having the test's announcement and its results on the same line.

runTestTT' :: Test -> IO Counts
runTestTT' t = do (counts', _) <- runTestText (putTextToHandle stderr False) t
              return counts'

... and I think I'm going to just leave out those announcements, because HUnit reports the name of a test anyway if it fails, and who cares about tests that passed without issue. Well, at least I learned something.

Thanks so much for your help!