Document non-react testing utility - createMockClient

[jeremybini]

We have a mixed application that uses Apollo Client for both React and non-react code.

Testing React code is easy and convenient given the exported and documented MockedProvider utility, however, for our non-react code, there is no clear documentation on the correct way to test usages of the client.

I noticed that apollo client exports a mock client from the testing directory

import { createMockClient } from '@apollo/client/testing'

Documenting and supporting this library as a valid form of testing the client would be very useful for those of us testing non-react code; additionally, it would alleviate the need for our own mock implementations or 3rd party libraries.

[jpvajda]

I'm linking this to a umbrella issue about testing in the apollo client repo apollographql/apollo-client#9738