Allow response mocking for unit tests

[phortx]

This plugin should provide some tools to allow the simple unit testing of components which interact with the GraphQL API.

[phortx]

I'm thinking of something like

import { clearORMStore, mock } from '@vuex-orm/plugin-graphql/test-utils';

clearORMStore();

mock('fetch', { variables: { id: 15 }}).for(User).andReturn({ name: 'Charlie Brown' });
mock('mutate', { mutation: 'upvote', variables: { id: 42 } }).for(Post).andReturn({ name: 'Example Post', upvotes: 4 });

to mock a User.fetch(15).

[phortx]

Released with v1.0.0-RC.18, see https://vuex-orm.github.io/plugin-graphql/guide/testing/