Add a test for #984

testsuite/tests/parser/should_fail/T984.hs

+
+module T984 where
+
+f _ = do
+        x <- computation
+        case () of
+                _ ->
+                        result <- computation
+                        case () of () -> undefined

testsuite/tests/parser/should_fail/T984.stderr

+
+T984.hs:6:9:
+    Parse error in pattern: case () of { _ -> result }
+    Possibly caused by a missing 'do'?

testsuite/tests/parser/should_fail/all.T

@@ -76,3 +76,4 @@ test('ParserNoLambdaCase', if_compiler_lt('ghc', '7.5', skip), compile_fail, [''
 test('ParserNoMultiWayIf', if_compiler_lt('ghc', '7.5', skip), compile_fail, [''])
 
 test('T5425', normal, compile_fail, [''])
+test('T984', normal, compile_fail, [''])