Is your feature request related to a problem? Please describe.
Apollo for React has https://www.apollographql.com/docs/react/development-testing/testing/, which implements a MockLink in here: https://github.com/apollographql/apollo-client/blob/master/src/utilities/testing/mocking/MockedProvider.tsx (this is the link to the apollo v3 version, but it's similar for v2)

Adding that unfortunately doesn't work nicely since it has a dependency on React.

Describe the solution you'd like

I'd like to add an "official" MockLink in vue-apollo, or a way to create a testing client which you can use.

Something like import { createMockClient } from '@vue/apollo-testing' would be ideal.

Describe alternatives you've considered
I've copied the code from MockLink into an own package so it doesn't depend on React, which works well internally at @sumcumo. Additionally, there is also https://github.com/Mike-Gibson/mock-apollo-client/, which works in a different way than the MockLink.
Suggesting stubbing apollo in tests is in my opinion a bad practise, see https://kentcdodds.com/blog/stop-mocking-fetch

This applies for both, v3 and v4.

I'm happy to work on it to provide a PR for both, v3 and v4, as well, if you think that it would belong to this library.