test: replace internals with public API

Remove instances where `--expose-internals` is used to gain access to
buffer.kStringMaxLength. The property is availalble without a flag. It
is undocumented but the same as the documented
`buffer.constants.MAX_STRING_LENGTH` so use that. (We even have a test
that confirms that they are the same value.)

PR-URL: https://github.com/nodejs/node/pull/25309


Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>