Document testing utils

[JulienPradet]

In a recent Pull Request, I've added a mock implementation of a dataloader to test a certain behavior of one of my loader. We should document these kind of mock/helpers functions dedicated to testing.

@real34 mentioned that we could add a Testing section containing these kinds of things.

So let's do this! :)

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A few notes about the mock implementation of a dataloader :

import {
  createLoader,
  hasMockDataloaderCache,
  resetAllMockDataloaders
} from "src/server/core/cache/strategies/mock.js";

• createLoader behaves exactly like the memoize.createLoader
• hasMockDataloaderCache(key, id) lets you know if a certain value is already cached for a particular dataloader
• resetAllMockDataloaders resets all your dataloaders (you usually want to call this function before/after each test.