Use getIntrospectionQuery rather than introspectionQuery constant.

The next major version of `graphql-js` deprecates the `introspectionQuery`
constant (thanks to graphql/graphql-js#2124) that
was previously leveraged in Apollo Client tests for obtaining the
introspection query (to be clear, this is the query used to introspect a
server, not the result of the introspection itself) in one particular test.

This deprecation isn't entirely surprising since the `getIntrospectionQuery`
utility method has existed since `[email protected]` and is the recommended way
to obtain this query in more recent versions of `graphql`, so this changes
that test to use that new method.

I discovered this while investigating our compatibility with the most
recently published `[email protected]` on `apollo-server`,
`apollo-tooling` and `apollo-client`.  Other than this, I didn't encounter
any test failures when updating the root `graphql` dev-dependency to
`[email protected]`!

Deprecated by: graphql/graphql-js#2124
In Response to: graphql/graphql-js#2303
Relates to: apollographql/apollo-server#3649
Relates to: apollographql/apollo-tooling#1743

src/__tests__/local-state/general.ts

@@ -1,6 +1,6 @@
 import gql from 'graphql-tag';
 import { DocumentNode, GraphQLError } from 'graphql';
-import { introspectionQuery } from 'graphql/utilities';
+import { getIntrospectionQuery } from 'graphql/utilities';
 
 import { Observable } from '../../utilities/observables/Observable';
 import { ApolloLink } from '../../link/core/ApolloLink';
@@ -71,7 +71,7 @@ describe('General functionality', () => {
 
   it('should not interfere with server introspection queries', () => {
     const query = gql`
-      ${introspectionQuery}
+      ${getIntrospectionQuery()}
     `;
 
     const error = new GraphQLError('no introspection result found');