Extracted repetitive assertions

Editor assessment

The `test(reactivity)` header is well scoped, and consolidating repetitive assertions is a legitimate test-suite cleanup, but two blemishes mar the subject. `extracted` uses past tense where imperative mood is required — `extract repetitive assertions` — and a stray double space appears before `(#554)`, cosmetic sloppiness some linters flag. Even so the header stays under 60 characters, so length is no issue. The absent body is tolerable for a test-only change, though one line naming which assertion groups were consolidated would help future maintainers. No runtime impact, no breaking markers; patch-level and machine-parseable. Correct intent, slightly careless wording.