[1/N] Refactor nightly test structure (#5479)

### What this PR does / why we need it?
This patch is a series of refactoring actions, including clarifying the
directory structure of nightly tests, refactoring the config retrieval
logic, and optimizing the workflow, etc. This is the first step:
refactoring the directory structure of nightly to make it more readable
and logical.

- vLLM version: v0.13.0
- vLLM main:
5326c89803

Signed-off-by: wangli <wangli858794774@gmail.com>

.github/workflows/nightly_test_a2.yaml

@@ -51,13 +51,13 @@ jobs:
         test_config:
           - name: qwen3-32b
             os: linux-aarch64-a2-4
-            tests: tests/e2e/nightly/models/test_qwen3_32b.py
+            tests: tests/e2e/nightly/single_node/models/test_qwen3_32b.py
           - name: qwen3-32b-in8-a2
             os: linux-aarch64-a2-4
-            tests: tests/e2e/nightly/models/test_qwen3_32b_int8.py
+            tests: tests/e2e/nightly/single_node/models/test_qwen3_32b_int8.py
           - name: test_custom_op
             os: linux-aarch64-a2-1
-            tests: tests/e2e/nightly/ops
+            tests: tests/e2e/nightly/single_node/ops/singlecard_ops
     uses: ./.github/workflows/_e2e_nightly_single_node.yaml
     with:
       vllm: v0.13.0