test: increase coverage for Histogram

1. custom inspect test case
Refs: https://coverage.nodejs.org/coverage-18e4f405b14b26f9/lib/internal/histogram.js.html#L56

2. tests that RecordableHistogram is impossible to construct manually
Refs: https://coverage.nodejs.org/coverage-18e4f405b14b26f9/lib/internal/histogram.js.html#L132

PR-URL: https://github.com/nodejs/node/pull/38555


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>