Make structure "deformity" tests more robust

[ml-evs]

Following the discussion in #582:

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Annoyingly if I hadn't made the extra field vs field.name mistake, then our tests would have caught this.

One extra comment here, we could make the deformity tests more robust by adding the expected error message for all of them too, this would have caught it as the validation of Hill formula cases was failing because of being out of alphabetical order rather than Hill. This will be a bit laborious but I can raise an issue and try to find the time.

Originally posted by @ml-evs in #582 (comment)