test: improve api/encoding.cc coverage

Hi, Node fam' 👋

Context

I intend to improve the coverage of this file.

As I'm not familiar with the code base, I didn't update the whole file, I prefer to collect feedback first 👍

Gains

Before	After
https://coverage.nodejs.org/coverage-7900f6540ea8ffd3/cxxcoverage.api_encoding.cc.html	https://app.codecov.io/gh/nodejs/node/blob/9294affe152c64104baf35ac4f7d0d61ca174a38/src/api/encoding.cc