[Testing utilities] Performance test against a large schema

[alessbell]

Issue Description

We will likely need to introduce caching when creating the mock schema (see #11705 (comment)).

Link to Reproduction

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Reproduction Steps

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@apollo/client version

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[alessbell]

createMockSchema has performed well with the fairly large Spotify schema when testing with the Spotify Showcase. That combined with the fact that it will likely only be called once during test setup (unlike createProxiedSchema) is enough for me to justify closing this for now, until an actual performance problem is felt, at while point I'll be happy to re-open and investigate.

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