expose TypeScript types for apollo-server-testing client (#2871)

* add typings for test client

* Update CHANGELOG.md

* Update CHANGELOG.md

CHANGELOG.md

@@ -7,6 +7,7 @@ The version headers in this history reflect the versions of Apollo Server itself
 > The changes noted within this `vNEXT` section have not been released yet.  New PRs and commits which introduce changes should include an entry in this `vNEXT` section as part of their development.  When a release is being prepared, a new header will be (manually) created below and the the appropriate changes within that release will be moved into the new section.
 
 - `apollo-server-core`: Avoid duplicate `cacheControl` directives being added via `isDirectiveDefined`. [PR #2762](https://github.com/apollographql/apollo-server/pull/2762) (was reverted in 2.6.1 because previous implementation released in 2.6.0 broke passing in typedefs as a string)
+- `apollo-server-testing`: Add TypeScript types for `apollo-server-testing` client. [PR #2871](https://github.com/apollographql/apollo-server/pull/2871)
 - `apollo-server-plugin-response-cache`: Fix undefined property access attempt which occurred when an incomplete operation was received. [PR #2792](https://github.com/apollographql/apollo-server/pull/2792) [Issue #2745](https://github.com/apollographql/apollo-server/issues/2745)
 
 ### v2.6.5

packages/apollo-server-testing/src/createTestClient.ts

@@ -1,4 +1,4 @@
-import { ApolloServerBase } from 'apollo-server-core';
+import { ApolloServerBase, GraphQLResponse } from 'apollo-server-core';
 import { print, DocumentNode } from 'graphql';
 
 type StringOrAst = string | DocumentNode;
@@ -21,7 +21,12 @@ type Mutation = {
   operationName?: string;
 };
 
-export default (server: ApolloServerBase) => {
+export interface ApolloServerTestClient {
+  query: (query: Query) => Promise<GraphQLResponse>;
+  mutate: (mutation: Mutation) => Promise<GraphQLResponse>;
+}
+
+export default (server: ApolloServerBase): ApolloServerTestClient => {
   const executeOperation = server.executeOperation.bind(server);
   const test = ({ query, mutation, ...args }: Query | Mutation) => {
     const operation = query || mutation;

packages/apollo-server-testing/src/index.ts

@@ -1 +1,4 @@
-export { default as createTestClient } from './createTestClient';
+export {
+  default as createTestClient,
+  ApolloServerTestClient,
+} from './createTestClient';