Improve test filtering logic

[rzabarazesh]

This PR fixes two main issues:

Issue 1: Pytest Markers Were Dropped

Expected behavior:

# GPU test step:
pytest -v -s -m 'not cpu_test' v1/core/test_scheduler.py

# CPU test step:
pytest -v -s -m 'cpu_test' v1/core/test_scheduler.py

Actual behavior (broken):

# Both steps:
pytest -v -s v1/core/test_scheduler.py  # Missing markers!

This caused tests to run without proper marker filtering, potentially executing GPU tests on CPU-only infrastructure.

Issue 2: Orphaned Tests Ran in Wrong Steps

Tests in directories not explicitly covered by a step's commands (e.g., v1/attention/) would be included based solely on the broad tests/v1 dependency, even though the step's commands never test that directory.

Example:

• "V1 Test others" step tests: v1/core, v1/executor, v1/worker, etc.
• Does NOT test: v1/attention
• But tests/v1/attention/test_attention_backends.py would still be added to this step

Testing

Running on main -> Working normally
https://buildkite.com/vllm/ci/builds/35289/steps/canvas

Running on a problematic PR (vllm-project/vllm#26663) -> Fix working
https://buildkite.com/vllm/ci/builds/35290/steps/canvas