fix(expenses): enforce category name in error message

[marcaurele]

Expenses exercise

The instructions are displaying the name of the category in the error message of the last bullet point. The test case must validate that the category name is included in the error message.

See

• https://exercism.org/tracks/go/exercises/expenses

• go/exercises/concept/expenses/.docs/instructions.md

  Line 159 in 5e937bc

  // => 0, error(unknown category entertainment)

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[github-actions]

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