rpc: Fix implicit-integer-sign-change in verifychain

Avoid implicit-integer-sign-change in VerifyLoadedChainstate

This happens when checking all blocks (check_blocks=-1).

The UB in VerifyLoadedChainstate was inadvertently fixed in D14614 when the unsigned int parameters became int64_t.
This backport just adds the test coverage in rpc_blochain.py

This is a backport of core#24403 and core#24347

With UBSAN

ninja all check-all