fix: Union test mock generator

[calvincestari]

This PR:

1. Adds a target dependency to the ApolloTestSupport package.
2. Adds the generation of test mock extensions for Union types that conform to MockFieldValue.

Alternatives solutions for Union conforming to MockFieldValue

• Change the Union protocol declaration to add conformance to MockFieldValue; the extension then to implement the typealias can be in that file or elsewhere. The problem with this approach is that MockFieldValue references AnyMock and attempting to import ApolloTestSupport creates a circular dependency. Bringing the AnyMock declaration into ApolloAPI resolves build errors in Union.swift but makes Union unusable in ParentType because of the associated type requirement - Protocol 'Union' can only be used as a generic constraint because it has Self or associated type requirements.

[calvincestari]

@AnthonyMDev let's discuss this again in the morning but I think generating the union test mock extensions may be the best solution.

[AnthonyMDev]

Looks like you've got a test that needs updating, but otherwise I'm good to ship this!

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