Add solver quickcheck test

cabal-install/tests/UnitTests/Distribution/Client/Dependency/Modular/QuickCheck.hs

@@ -42,6 +42,14 @@ tests = [
                              ReverseOrder -> reverse targets
             in counterexample (showResults r1 r2) $
                isJust (resultPlan r1) === isJust (resultPlan r2)
+
+    , testProperty
+          "solvable without --independent-goals => solvable with --independent-goals" $
+          \(SolverTest db targets) reorderGoals solver ->
+            let r1 = solve reorderGoals (IndepGoals False) solver targets db
+                r2 = solve reorderGoals (IndepGoals True)  solver targets db
+             in counterexample (showResults r1 r2) $
+                isJust (resultPlan r1) `implies` isJust (resultPlan r2)
     ]
   where
     showResults :: Result -> Result -> String
@@ -53,6 +61,9 @@ tests = [
                ++ resultLog result
                ++ ["result: " ++ show (resultPlan result)]
 
+    implies :: Bool -> Bool -> Bool
+    implies x y = not x || y
+
 solve :: ReorderGoals -> IndepGoals -> Solver -> [PN] -> TestDb -> Result
 solve reorder indep solver targets (TestDb db) =
   let (lg, result) =