[Testing utilities] Custom fetch implementation via context

[alessbell]

Issue Description

@phryneas had a great idea here: instead of requiring users to use an test env-specific terminating link of SchemaLink, use a custom fetch implementation via context that effectively turns HttpLink into a SchemaLink.

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[alessbell]

@jerelmiller also had an idea here around setting a threshold (min/max) values for response timing

[jerelmiller]

Yes! To write it out, we want to mimic a real client/server interaction as much as possible, which means that there will always be variable latency in the request. We absolutely DON'T want the mocked request to be synchronous because a network request is always asynchronous (something we see too often in MockedProvider unfortunately). Allowing a min/max for response timing allows us to randomize the synthetic latency to better mimic a production environment. This also provides better guarantees that your tests aren't passing purely because the request timing is favorable.

[alessbell]

Completed via #11605

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