add test to ensure public resolvers can only be defined once per URL

x/data/server/features/msg_define_resolver.feature

@@ -15,6 +15,11 @@ Feature: Msg/DefineResolver
       When alice attempts to define a resolver with url "https://foo.bar"
       Then expect the error "a resolver with the same URL and manager already exists: unique key violation"
 
+    Scenario: public resolvers can only be defined once per URL
+      Given a public resolver is defined for the url "ipfs:"
+      When alice attempts to define a public resolver with url "ipfs:"
+      Then expect the error "a resolver with the same URL and manager already exists: unique key violation"
+
   Rule: Event is emitted
 
     Scenario: EventDefineResolver is emitted

x/data/server/msg_define_resolver_test.go

@@ -41,13 +41,29 @@ func (s *defineResolverSuite) AliceHasDefinedAResolverWithUrl(a string) {
 	require.NoError(s.t, err)
 }
 
+func (s *defineResolverSuite) APublicResolverIsDefinedForTheUrl(a string) {
+	err := s.server.stateStore.ResolverTable().Insert(s.ctx, &api.Resolver{
+		Url:     a,
+		Manager: nil,
+	})
+	require.NoError(s.t, err)
+}
+
 func (s *defineResolverSuite) AliceAttemptsToDefineAResolverWithUrl(a string) {
 	_, s.err = s.server.DefineResolver(s.ctx, &data.MsgDefineResolver{
 		Definer:     s.alice.String(),
 		ResolverUrl: a,
 	})
 }
 
+func (s *defineResolverSuite) AliceAttemptsToDefineAPublicResolverWithUrl(a string) {
+	_, s.err = s.server.DefineResolver(s.ctx, &data.MsgDefineResolver{
+		Definer:     s.alice.String(),
+		ResolverUrl: a,
+		Public:      true,
+	})
+}
+
 func (s *defineResolverSuite) BobAttemptsToDefineAResolverWithUrl(a string) {
 	_, s.err = s.server.DefineResolver(s.ctx, &data.MsgDefineResolver{
 		Definer:     s.bob.String(),