Fix sorted-set backlink test isolation (#4756)

The backlink consistency test reuses `myzset` left by the sorting
stresser. In the listpack case, those existing numeric members combine
with the new `Element-*` members and can exceed the configured 256-entry
limit, converting the set to btree before the encoding assertion.

Delete `myzset` at the start of the backlink test, as the neighboring
stress tests do, so it exercises only its own data and retains the
intended encoding.

Fixes #4751.

Validation on macOS ARM64:

- `make -j8`
- Deterministic reproduction using the existing sorting-stresser and
backlink test bodies with `srand(11988)`: the unmodified test fails with
`Expected 'btree' to match 'listpack'`; adding the cleanup makes both
tests pass.
- `./runtest --single unit/type/zset --clients 1` — 368 passed, 0
failed, including both encodings and the macOS memory-leak check.
- `git diff --check`

Signed-off-by: Roy Chen <rchen1386@gmail.com>
Signed-off-by: chky1386 <63842058+chky1386@users.noreply.github.com>