tests: cover Silent Payments label scanning #1898

The ctime test marks the scan key undefined, but generates its output for a scan public key created before later key mutations.
The scan therefore cannot find that output, leaving the successful-match path untested even though the test passes.

Regenerate the output for the actual scan key and prepend a nonmatching output for the label-batch scenario.
Remove the earlier scan-public-key creation and sender call because this setup replaces both.

The ctime test passes no label callback, so label-candidate serialization and batching never run while the scan key is marked undefined.

Check that every serialized 33-byte candidate is defined, count callback misses, and require one direct unlabeled output match.
For `k = 0`, the first output contributes two candidates that flush before the second output matches directly; for `k = 1`, both outputs miss and four candidates flush at the end.

The six callback calls prove both flush paths execute, `SECP256K1_CHECKMEM_CHECK` proves the candidate bytes are defined, and the result assertions prove the intended direct match is still found.
The six-call argument relies on `LABEL_BATCH_SIZE >= 2` (currently 8); at one, the first miss would flush immediately and no longer cover the pre-direct-match flush.