test,tools: refactor custom ESLint for readability

Refactor the test and the source for the `lowercase-name-for-primitive`
custom ESLint rule for readability.

PR-URL: https://github.com/nodejs/node/pull/21134


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>