Codechange: Simplify SetBitIterator

Use FindFirstBit and KillFirstBit, allowing simpler iterator equality
Add simple test

Add: Mock sprite cache intialization.

This fills up the sprite cache with SPR_OPENTTD_BASE + OPENTTD_SPRITE_COUNT zero-size sprites, to
allow GetSpriteSize() calls to continue from unit-tests.

Add: WindowDesc unit test to validate ini-key value.

ini-key must be present if WWT_DEFSIZEBOX or WWT_STICKYBOX is present.
This was previously enforced by a workflow, however that parsed the source
code with regex which turned out to be error-prone.

Codechange: add tests for GS <-> AdminPort JSON conversion (#11252)

While at it, fix a bug where booleans were made integers.