add tests

tests/core/snapshots/test-invalid-add-field.md_error.snap

@@ -0,0 +1,15 @@
+---
+source: tests/core/spec.rs
+expression: errors
+---
+[
+  {
+    "message": "Parsing failed because of invalid type: string \"{{.value.user.username}}\", expected a sequence",
+    "trace": [
+      "PostUser",
+      "@addField",
+      "path"
+    ],
+    "description": null
+  }
+]

tests/execution/test-invalid-add-field.md

@@ -0,0 +1,31 @@
+---
+error: true
+---
+
+# Test invalid add fields
+
+```graphql @config
+schema @server(port: 8000) @upstream(baseURL: "http://jsonplaceholder.typicode.com") {
+  query: Query
+}
+
+type Query {
+  postuser: [PostUser] @http(path: "/posts")
+}
+
+type PostUser @addField(name: "username", path: "{{.value.user.username}}") {
+  id: Int! @modify(name: "postId")
+  title: String!
+  userId: Int!
+  user: User @http(baseURL: "https://jsonplaceholder.typicode.com", path: "/users/{{.value.userId}}")
+}
+
+type User {
+  id: Int!
+  name: String!
+  username: String!
+  email: String!
+  phone: String
+  website: String
+}
+```