test: improve `lib/internal/webstreams/readablestream.js` coverage

This improve lib/internal/webstreams/readablestream.js coverage

Refs:

• https://coverage.nodejs.org/coverage-3a6b975981092213/lib/internal/webstreams/readablestream.js.html#L421
• https://coverage.nodejs.org/coverage-3a6b975981092213/lib/internal/webstreams/readablestream.js.html#L443
• https://coverage.nodejs.org/coverage-3a6b975981092213/lib/internal/webstreams/readablestream.js.html#L476
• https://coverage.nodejs.org/coverage-3a6b975981092213/lib/internal/webstreams/readablestream.js.html#L949
• https://coverage.nodejs.org/coverage-3a6b975981092213/lib/internal/webstreams/readablestream.js.html#L1113