Some tests added in r141471 require compound words such as "upper case" to be corrected to "uppercase".
We should support this in DumpRenderTree somehow.
I reckon this is more global issue rather than Mac only.
One option that comes to mind is until we use unified text checker that passes whole content to spell check (mostly needed by grammar checking) we could treat such phrases as exceptions and just mark them as misspellings. In addition, defining replacements for them and filling TextCheckingResult::replacement will guarantee proper corrections.