Codechange: add tests for GS <-> AdminPort JSON conversion

[TrueBrain]

Motivation / Problem

In #11251 I want to change our custom JSON conversions to nlohmann. But to know I did a good job, I first need tests.

Description

Of course during the writing of the tests you find small errors. Fixed one in #11250, and in this PR I fix another one: when loading in JSON, booleans were converted to integers. No biggy, as in Squirrel those two are interchangeable for most part.

There is one bug remaining that the tests detected in the current implementation: when loading JSON it fails to load in an empty table. This will be addressed when converting to nlohmann.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[rubidium42]

Looks okay to me, except maybe an idea for an extra test that could be added.