Add test for introspection as subscription root

Add a pending test checking that an introspection field cannot be
subscription root.

tests/Language/GraphQL/Validate/RulesSpec.hs

@@ -18,7 +18,7 @@ import qualified Language.GraphQL.AST.DirectiveLocation as DirectiveLocation
 import qualified Language.GraphQL.Type.In as In
 import qualified Language.GraphQL.Type.Out as Out
 import Language.GraphQL.Validate
-import Test.Hspec (Spec, context, describe, it, shouldBe, shouldContain)
+import Test.Hspec (Spec, context, describe, it, shouldBe, shouldContain, xit)
 import Text.Megaparsec (parse, errorBundlePretty)
 
 petSchema :: Schema IO
@@ -206,6 +206,18 @@ spec =
                         }
                  in validate queryString `shouldContain` [expected]
 
+            xit "rejects an introspection field as the subscription root" $
+                let queryString = "subscription sub {\n\
+                        \  __typename\n\
+                        \}"
+                    expected = Error
+                        { message =
+                            "Subscription \"sub\" must select only one top \
+                            \level field."
+                        , locations = [AST.Location 1 1]
+                        }
+                 in validate queryString `shouldContain` [expected]
+
             it "rejects multiple subscription root fields coming from a fragment" $
                 let queryString = "subscription sub {\n\
                         \  ...multipleSubscriptions\n\