GHCi doesn't run computations in a new thread

A broken QuickCheck property cause ghci to panic after
reloading the module. Seen in stable and head branch.

paprika$ ghci T.hs
*Main> do_test
                     test : *                         
      (0)
*Main> :reload
ghc-6.5: panic! (the `impossible' happened, GHC version
6.5):
        <<loop>>

Please report it as a compiler bug to
glasgow-haskell-bugs@haskell.org,
or http://sourceforge.net/projects/ghc/.

Changing the property to check for empty lists causes
the test to pass, and reload to work fine.

-- Don Stewart