test: add whatwg-encoding TextDecoder custom inspection with showHidden

These tests ensure hidden fields are shown when inspecting with
`showHidden` and that passing negative `depth` prints simplified value.

Co-authored-by: Robin Drexler <drexler.robin@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/24166


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>