named_args_spec: add tests for unreadable formulae/casks

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Follow-up to #10405

I was going to include this in a different PR, but I thought I should first open this one, where I'm only adding tests. Once this is merged, the intent behind the PR I plan open will hopefully be more clear

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Review period will end on 2021-01-27 at 02:48:14 UTC.

[MikeMcQuaid]

👏🏻 ❤️

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Review period ended.