test: add test case for reproducible bug

[li-kai]

This adds a test case for apollographql/react-apollo#3425. This is intended to fail as it is trying to reproduce the bug.

Context

Despite an update occurring, this bug causes the query's loading to remain true. It happens when we:

1. Make a query with variable a.
2. Skip a query.
3. Make a query with variable b. This query's result is the same as step 1's result.

Tracing this bug

This is roughly the call stack for step 3 mentioned above:

1. useQuery
2. useBaseQuery
3. QueryData.execute
4. QueryData.updateObservableQuery
5. ObservableQuery.setOptions
6. ObservableQuery.setVariables
7. QueryManager.fetchQuery
8. QueryManager.fetchRequest
9. observable's map and complete callbacks are called
10. ObservableQuery's observer's next is called
11. ObservableQuery.isDifferentFromLastResult

Note that since the new and old data is the same despite having different query variables, step 11 ObservableQuery's isDifferentFromLastResult will return false.

This causes the ObservableQuery's observers' to not be called. QueryData's currentObservable.subscription does not trigger and the component does not update.

Fixing the bug

This is where I need help. I am not sure what would be the proper fix. Here a few ideas:

• Add previousVariables to ObservableQuery and compare them in isDifferentFromLastResult
• Mark the data as stale when a skip happens
• Trigger something on step 9's complete

[apollo-cla]

@li-kai: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[li-kai]

Solved by #6120