Test standalone deriving for GADTs

testsuite/tests/ghc-regress/deriving/should_compile/T3012.hs

+{-# LANGUAGE GADTs, StandaloneDeriving #-}
+
+module T3012 where
+
+data T a where
+    Foo :: T Int
+    Bar :: T Bool
+
+deriving instance Show (T a)
+

testsuite/tests/ghc-regress/deriving/should_compile/all.T

@@ -19,6 +19,7 @@ test('deriving-1935', normal, compile, [''])
 test('T2378', normal, compile, [''])
 test('T2856', normal, compile, [''])
 test('T3057', if_compiler_lt('ghc', '6.11', expect_fail), compile, [''])
+test('T3012', normal, compile, [''])
 test('drv-functor1',
      if_compiler_lt('ghc', '6.11', expect_fail),
      compile,