Fix monero-sys test expectations

[himanalot]

Summary

• expose Wallet::verifySignedMessage through monero-sys so signed-message tests assert signature validity instead of deterministic bytes
• update sign_message to verify spend-key, view-key, explicit-address signatures and reject a tampered message
• send the simple test self-transfer to a newly-created subaddress, since Monero does not produce usable tx keys for primary-address self-transfers

Verification

• cargo test -p monero-sys --test sign_message --no-fail-fast
• cargo test -p monero-sys --test simple --no-fail-fast
• cargo fmt -p monero-sys -- --check
• git diff --check

Addresses the monero-sys slice of #724.

/claim #724