Allow resolvers to be passed into MockedProvider and set a default

After the changes in apollographql/apollo-client#4499,
when `MockedProvider` is currently used it doesn't have any
`resolvers` set. This means `@client` directives are passed into
the link chain. Since we don't currently allow people to modify the
link chain that `MockedProvider` uses, it's safe to assume people won't
want this behaviour, as they can't test things like `apollo-link-state`
this way. This commit sets a default `resolvers` value of `{}` to
enable AC 2.5's new local state handling, and opens the door for
passing custom resolvers into the `MockedProvider` via props.

Helps fix apollographql/fullstack-tutorial#73.

src/test-utils.tsx

@@ -1,6 +1,6 @@
 import * as React from 'react';
 import ApolloClient from 'apollo-client';
-import { DefaultOptions } from 'apollo-client';
+import { DefaultOptions, Resolvers } from 'apollo-client';
 import { InMemoryCache as Cache } from 'apollo-cache-inmemory';
 
 import { ApolloProvider } from './index';
@@ -13,6 +13,7 @@ export interface MockedProviderProps<TSerializedCache = {}> {
   addTypename?: boolean;
   defaultOptions?: DefaultOptions;
   cache?: ApolloCache<TSerializedCache>;
+  resolvers?: Resolvers;
 }
 
 export interface MockedProviderState {
@@ -27,11 +28,18 @@ export class MockedProvider extends React.Component<MockedProviderProps, MockedP
   constructor(props: MockedProviderProps) {
     super(props);
 
-    const { mocks, addTypename, defaultOptions, cache } = this.props;
+    const {
+      mocks,
+      addTypename,
+      defaultOptions,
+      cache,
+      resolvers = {},
+    } = this.props;
     const client = new ApolloClient({
       cache: cache || new Cache({ addTypename }),
       defaultOptions,
       link: new MockLink(mocks || [], addTypename),
+      resolvers,
     });
 
     this.state = { client };