test: improve buffer coverage

Refs: https://coverage.nodejs.org/coverage-18e4f405b14b26f9/lib/buffer.js.html#L188 https://coverage.nodejs.org/coverage-18e4f405b14b26f9/lib/buffer.js.html#L198 https://coverage.nodejs.org/coverage-18e4f405b14b26f9/lib/buffer.js.html#L240 https://coverage.nodejs.org/coverage-18e4f405b14b26f9/lib/buffer.js.html#L607 https://coverage.nodejs.org/coverage-18e4f405b14b26f9/lib/buffer.js.html#L651