Validate the subscription root

…not to be an introspection field.

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, xit)
+import Test.Hspec (Spec, context, describe, it, shouldBe, shouldContain)
 import Text.Megaparsec (parse, errorBundlePretty)
 
 petSchema :: Schema IO
@@ -206,14 +206,14 @@ spec =
                         }
                  in validate queryString `shouldContain` [expected]
 
-            xit "rejects an introspection field as the subscription root" $
+            it "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."
+                            "Subscription \"sub\" must select exactly one top \
+                            \level field, which must not be an introspection field."
                         , locations = [AST.Location 1 1]
                         }
                  in validate queryString `shouldContain` [expected]