🐛 Bug: [the-shape-of-types] MockHorror class fails type checking in solution tests

[JoshuaKGoldberg]

Bug Report Checklist

• [X] I have tried restarting my IDE and the issue persists.
• [X] I have pulled the latest main branch of the repository.
• [X] I have searched for related issues and found none that matched my issue.
• [X] This is the appropriate issue form for the bug I would like to report.

Expected

npm run test:solutions should pass in all projects, including projects/classes/the-shape-of-types.

Actual

Argument of type 'MockHorror' is not assignable to parameter of type 'Horror & Horror'.
  Type 'MockHorror' is missing the following properties from type 'Horror': #consumed, #consume, doBattle, getPowerts(2345)

Impacted Project

the-shape-of-types > src/index.test.ts

Additional Info

When testing solutions, this happens:

1. solution.ts is copied to index.ts
2. The top of index.test.ts is rewritten to:

   const { Demon, Horror, Sorcerer } = process.env.TEST_SOLUTIONS
       ? index
       : // In theory, it would be nice to not have to apply this cast
         // In practice, TypeScript's structural typing does not play well with # privates
         // See https://github.com/LearningTypeScript/projects/issues/183
         (index as unknown as typeof solution);

3. tsc

At the end, the Horror class is known by TypeScript to be either the import from index or solution. And because Horror has # privates, the two classes aren't assignable to each other.