Add :doc test case for duplicate record fields

Tests that the output of the `:doc` command is correct for duplicate
record fields defined using -XDuplicateRecordFields.

testsuite/tests/ghci/scripts/ghci065.hs

@@ -5,6 +5,7 @@
 -- this test is constructed with simple text (without markup) only.
 --
 
+{-# LANGUAGE DuplicateRecordFields #-}
 module Test where
 
 -- | This is the haddock comment of a data declaration for Data1.
@@ -21,6 +22,18 @@ newtype Data4 =
   -- | This is the haddock comment of a data constructor for Data4.
   Data4 { getData4 :: Int }
 
+data DupeFields1 =
+  DF1 { dupeField :: Int -- ^ This is the first haddock comment of a duplicate record field.
+      }
+
+data DupeFields2 =
+  DF2 { dupeField :: Int -- ^ This is the second haddock comment of a duplicate record field.
+      }
+
+data DupeFields3 =
+  DF3 { dupeField :: Int -- No haddock
+      }
+
 -- | This is the haddock comment of a function declaration for func1.
 func1 :: Int -> Int -> Int
 func1 x y = x + y

testsuite/tests/ghci/scripts/ghci065.script

@@ -7,6 +7,7 @@
 :doc Val2b
 :doc Data3
 :doc Data4
+:doc dupeField
 
 :doc func1
 :doc func2

testsuite/tests/ghci/scripts/ghci065.stdout

@@ -3,6 +3,9 @@
  This is the haddock comment of a data value for Val2b
  This is the haddock comment of a data declaration for Data3.
  This is the haddock comment of a data constructor for Data4.
+ This is the second haddock comment of a duplicate record field.
+
+ This is the first haddock comment of a duplicate record field.
  This is the haddock comment of a function declaration for func1.
 <has no documentation>
  This is the haddock comment of a function declaration for func3.