Added test cases related to database operations

[wu-zhao-min]

Added the processing of boundary conditions for the new database, the complete operation process of cursors, and the test cases for database statistics information.

[gburd]

Thanks for the contribution, and for thinking about boundary/cursor/stat coverage — that kind of testing is welcome.

Unfortunately I can't merge this as-is: it won't compile. test/c/test_db185.c exercises the DB 1.85 compatibility API (it #includes db_185.h and dbp is the 1.85 DB). That handle is defined in src/dbinc/db_185.in and has only these methods:

typedef struct __db {
    DBTYPE type;
    int (*close)(struct __db *);
    int (*del)(const struct __db *, const DBT *, u_int);
    int (*get)(const struct __db *, const DBT *, DBT *, u_int);
    int (*put)(const struct __db *, DBT *, const DBT *, u_int);
    int (*seq)(const struct __db *, DBT *, DBT *, u_int);
    int (*sync)(const struct __db *, u_int);
    void *internal;
    int (*fd)(const struct __db *);
} DB;

So the new code can't build here:

• dbp->cursor(...) and dbp->stat(...) — no such members on the 1.85 DB.
• cursor->c_get/c_put/c_close (DBC), and constants DB_NEXT, DB_SET, DB_KEYFIRST, DB_NOTFOUND, DB_BTREE_STAT/DB_HASH_STAT/DB_RECNO_STAT — these are the modern API, not in db_185.h (the 1.85 API iterates with seq() + R_NEXT/R_FIRST and has no cursors or stat).

A couple of other issues to flag regardless:

• char large_key[1024*1024] is a 1 MB stack array (stack overflow risk).
• In test_db_cursor, sprintf((char *)key.data, ...) writes into the buffer returned by c_get, which is owned by Berkeley DB — that's a write into library-owned memory.

Two good ways forward:

1. Best: add these cursor/stat/boundary tests against the modern API — e.g. a new test/c/ program (or extend one that uses db.h/DB_ENV), where DB->cursor, DBC->get, and DB->stat exist. That's where this coverage belongs.
2. If you specifically want to extend the 1.85 path, use only get/put/del/seq/sync with the R_* flags, and keep buffers caller-owned.

Happy to review a revised version. Closing for changes, not merging.