test: validate webstream encoder/decoder inspector

This validates that inspcetor outputs of TextEncoderStream & TextDecoderStream in internal/webstreams.

ref:

• https://coverage.nodejs.org/coverage-24adba675179ebba/lib/internal/webstreams/encoding.js.html#L98
• https://coverage.nodejs.org/coverage-24adba675179ebba/lib/internal/webstreams/encoding.js.html#L98