[serokell#208] redundant constraint test

Problem: in `ghc-8.6.3` your code can produce `redundant constraint` warning
if you derive `Container` for newtype, but in `ghc-9.0.2` and newer
no warnings would be produced.

Solution: added test, that should be compiled with `ghc-9.0.2` and newer
without warnings.

test/Test/Universum/Issue208.hs

@@ -0,0 +1,23 @@
+{-# LANGUAGE
+    GeneralizedNewtypeDeriving
+  , UndecidableInstances
+  , FlexibleContexts
+  , GADTs
+  , DerivingStrategies
+  , CPP
+#-}
+module Test.Universum.Issue208
+  () where
+
+#if __GLASGOW_HASKELL__ >= 900
+
+import Universum (Container)
+
+-- In ghc-8.6.3 this code will produce a @redundant constraint@ warning.
+-- In ghc-9.0.2 and newer no warnings would be produced.
+-- Issue #208: https://github.com/serokell/universum/issues/208
+
+newtype Test = Test [Int]
+    deriving newtype (Container)
+
+#endif

universum.cabal

@@ -121,7 +121,9 @@ test-suite universum-test
   hs-source-dirs:      test
   main-is:             Spec.hs
 
-  other-modules:       Test.Universum.Property
+  other-modules:
+                       Test.Universum.Issue208
+                       Test.Universum.Property
 
   build-depends:       universum
                      , bytestring