graphql: fix spurious travis failure (ethereum#22166)

The tests sometimes failed with certain go versions because
the behavior of http.Server.Shutdown changed over time. A bug
that was fixed in Go 1.15 could cause active connections on unrelated
servers to close unexpectedly. This is fixed by avoiding use of the
same port number in all tests.

graphql/graphql_test.go

@@ -131,7 +131,7 @@ func TestGraphQLBlockSerialization(t *testing.T) {
 			code: 200,
 		},
 	} {
-		resp, err := http.Post(fmt.Sprintf("http://%s/graphql", "127.0.0.1:9393"), "application/json", strings.NewReader(tt.body))
+		resp, err := http.Post(fmt.Sprintf("%s/graphql", stack.HTTPEndpoint()), "application/json", strings.NewReader(tt.body))
 		if err != nil {
 			t.Fatalf("could not post: %v", err)
 		}
@@ -156,7 +156,7 @@ func TestGraphQLHTTPOnSamePort_GQLRequest_Unsuccessful(t *testing.T) {
 		t.Fatalf("could not start node: %v", err)
 	}
 	body := strings.NewReader(`{"query": "{block{number}}","variables": null}`)
-	resp, err := http.Post(fmt.Sprintf("http://%s/graphql", "127.0.0.1:9393"), "application/json", body)
+	resp, err := http.Post(fmt.Sprintf("%s/graphql", stack.HTTPEndpoint()), "application/json", body)
 	if err != nil {
 		t.Fatalf("could not post: %v", err)
 	}
@@ -177,21 +177,21 @@ func TestGraphQLHTTPOnSamePort_GQLRequest_Unsuccessful(t *testing.T) {
 func createNode(t *testing.T, gqlEnabled bool) *node.Node {
 	stack, err := node.New(&node.Config{
 		HTTPHost: "127.0.0.1",
-		HTTPPort: 9393,
+		HTTPPort: 0,
 		WSHost:   "127.0.0.1",
-		WSPort:   9393,
+		WSPort:   0,
 	})
 	if err != nil {
 		t.Fatalf("could not create node: %v", err)
 	}
 	if !gqlEnabled {
 		return stack
 	}
-	createGQLService(t, stack, "127.0.0.1:9393")
+	createGQLService(t, stack)
 	return stack
 }
 
-func createGQLService(t *testing.T, stack *node.Node, endpoint string) {
+func createGQLService(t *testing.T, stack *node.Node) {
 	// create backend
 	ethConf := &eth.Config{
 		Genesis: &core.Genesis{