Fix build failure against dev-haskell/test-framework-0.8

[147 of 147] Compiling Main             ( src/test.hs, dist/build/darcs-test/darcs-test-tmp/Main.o )

src/test.hs:100:30:
    No instance for (Typeable ShellTest) arising from a use of `Test'
    Possible fix: add an instance declaration for (Typeable ShellTest)
    In the expression: Test (file ++ " (" ++ show fmt ++ ")")
    In the expression:
      Test (file ++ " (" ++ show fmt ++ ")") $ ShellTest fmt file tdir dp
    In an equation for `shellTest':
        shellTest dp fmt tdir file
          = Test (file ++ " (" ++ show fmt ++ ")")
            $ ShellTest fmt file tdir dp
diff --git a/src/test.hs b/src/test.hs
index 39adcc1..9687074 100644
--- a/src/test.hs
+++ b/src/test.hs
@@ -49,6 +49,7 @@ data ShellTest = ShellTest { format :: Format
                            , testdir  :: Maybe FilePath -- ^ only if you want to set it explicitly
                            , _darcspath :: FilePath
                            }
+    deriving Typeable
 
 runtest' :: ShellTest -> FilePath -> ShIO Result
 runtest' (ShellTest fmt _ _ dp) srcdir =