Fix problem with test files not added to distribution tarball

haddock.cabal

@@ -48,6 +48,9 @@ extra-source-files:
   haddock.spec
   haskell.vim
   src/haddock.sh
+-- The test files shouldn't have to go here, but the source files for
+-- the test-suite stanzas don't get picked up by `cabal sdist`.
+  tests/html-tests/runtests.hs
 
 data-files:
   html/frames.html