NimbusReviewSerializer tests are repetitive

[freshstrangemusic]

Almost every NimbusReviewSerializer test includes the following snippet:

        serializer = NimbusReviewSerializer(
            experiment,
            data=NimbusReviewSerializer(experiment, context={"user": self.user}).data,
            context={"user": self.user},
        )

We should add a helper to the testcase for this, which would remove ~500 lines and make tests easier to write.

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