Strengthen mirrored truncate verification to only run after fully successful mirror truncates. Here are the details.

• mirrored_truncate is currently set as soon as we see a TRUNCATE on a mirrored table, before we know whether all mirror truncates have succeeded.
• If a mirror returns WT_ROLLBACK and the transaction is rolled back, we can still call wts_verify_mirrored_truncate, which assumes a successful mirrored truncate and may report misleading mirror-verify failures.
• This change moves the mirrored_truncate assignment to after the mirror loop and guards it with op == TRUNCATE && ret == 0, so verification only runs when the mirrored truncate actually committed on all tables.