[Test] Fix UT for EPLBController (#15283)

### What this PR does / why we need it?

This PR updates the unit tests for `AscendEPLBController` to align with
the new `prepare_forward` interface instead of the deprecated `step`
method. This ensures that the unit tests correctly verify the load
collection phase filtering and step recording behavior.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

The patch itself consists of unit test updates. The updated unit tests
in `tests/ut/worker/v2/test_eplb_controller.py` were executed and
passed.

- vLLM main:
https://github.com/vllm-project/vllm/commit/ba07e4a48fc951300d97eb506217dd530583dea3

Signed-off-by: Spicy-Stick <873805887@qq.com>