843b15d0

Construct test fixtures with shared generic helpers

Port MustCreate and MustCreate2 from refactoring-english-feedback. Both
helpers use the supplied constructor and fail the calling test when it
rejects the input, rather than requiring a wrapper for each value type.

Migrate test callers and remove redundant test-only constructor helpers.
Keep panic helpers that still have production callers. Cover successful
construction and fatal error handling for both generic helpers.

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