Test VarDCT block smaller/larger than allowed by spec

[foolip]

This is the only remaining part for VarDCT on the testing roadmap.

@veluca93 I think you have argued this isn't really useful, can you remind me why? (We could change the roadmap instead.)

[veluca93]

I have to admit I am not sure what this even refers to :-)

[veluca93]

To clarify: I don't understand what this refers to because out-of-spec DCT sizes just can't be represented in JPEG XL files, so I don't know what should be tested here.

[foolip]

Thanks @veluca93! I added this to the roadmap on the assumption that it was possible to represent but there was a requirement that implementations not support it. If it's actually impossible to represent because of how block sizes are encoded, then there's nothing to test. I'll update the investigation roadmap.