Fix (by adding an assert and testing) cases where cores return an undeclared sysID

[YoshiRulz]

prior comment

From #3635 (comment):

How about this:

• RomLoader never sets SGB on any rom (since any GB rom could be used with SGB, they should all be detected as the same, and GB/GBC is the better choice)
• There is no core preference for SGB, only GB and maybe GBC
• Cores are perfectly honest with setting IEmulator.SystemId (not necessarily matching the one passed in, since a sync setting can choose between GB/GBC/SGB)
• SystemId always matches a [CoreConstructor]—if it doesn't make sense to have that ctor, like with SGB which RomLoader will never use, it can simply be private and call something like this(default)

I think that would give the consistency I'm after while being simpler for regular users.

Later comment from #3960 (comment):
The value returned from IEmulator.SystemId should always match one of the [CoreConstructor]s. There should not be any case where the core "knows better" and swaps sysID based on the rom, for example—it is the frontend's responsibility to identify roms—but for the time being those hacks can remain and the core can simply add another attribute, and possibly a dummy ctor.