```
The VERIFY macro turns on various paranoid consistency checks, but the
complete functionality should still be tested without it.
This also adds a couple of static test points for extremely small split
inputs/outputs. The existing bounds vectors already check extremely
large outputs.
```
Partial backport 7/11 of secp256k1 PR830:
https://github.com/bitcoin-core/secp256k1/pull/830/commits/ebad8414b0e68041568d0b5ebe0bd395dbfbed9e
Depends on D8042.