Fix createTestClient types

[borekb]

The problem

I was following this documentation which shows code like this:

const { createTestClient } = require('apollo-server-testing');
const { query, mutate } = createTestClient(server);

query({
  query: GET_USER,
  variables: { id: 1 }
});

When I tried that, TypeScript complained about the variables object. This can be also replicated by opening the createTestClient.test.ts file:

The fix

To fix this, I've updated the typing of the test function in 62bea35. With this and another fix in 4ccc129, the createTestClient.test.ts file is clear of any errors.

VSCode experience

The experience when typing a call site code seems to be good. Initially, it offer all three properties:

Once a query or a mutation is chosen, it narrows down suggestions to only variables:

Providing both is an error:

Adding variables is valid:

It is done using function overloads which I generally don't like very much but maybe here, it's a good use case for them.

Is the change from ...rest to variables OK?

The code now only allows (strongly-typed) variables while previously, it worked with ...rest. Is this change OK? I think it should be as the tests don't show any other use case but it's a change in functionality so I thought I'd rather ask.

TODO

• (probably not necessary?) Update CHANGELOG.md with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass. (I'm not quite sure how to do that only for the apollo-server-testing but I've ran all tests in this package and they are green.)

[borekb]

#2307 seems to be resolving the same issue, in another way, technically, but I think we can close this.