tests: Use an empty subscriptionsPath when asserting subscriptions …

…eq false.

This fixes a TypeScript `TypeError` which was encountered after the
`@types/ws` typings were updated[[1]] to more correctly identify valid URLs.

The error exhibited on CI[[2] was:

TypeError [ERR_INVALID_URL]: Invalid URL: ws://localhost:6666undefined

This had been previously reverted[[3]] but was auto-merged again for a
reason I don't quite have time to investigate right now, though I suspect
it's something I'll be able to prevent in the future with a bit more care.

[1]: #1992
[2]: https://circleci.com/gh/apollographql/apollo-server/22416
[3]: #2006

cc @martijnwalraven

packages/apollo-server-integration-testsuite/src/ApolloServer.ts

@@ -978,7 +978,7 @@ export function testApolloServer<AS extends ApolloServerBase>(
           });
         });
       });
-      it('disables subscritpions when option set to false', done => {
+      it('disables subscriptions when option set to false', done => {
         const typeDefs = gql`
           type Query {
             "graphql-js forces there to be a query type"
@@ -1025,7 +1025,7 @@ export function testApolloServer<AS extends ApolloServerBase>(
           }
 
           const client = new SubscriptionClient(
-            `ws://localhost:${port}${server.subscriptionsPath}`,
+            `ws://localhost:${port}${server.subscriptionsPath || ''}`,
             {},
             WebSocket,
           );