[Test] Unify L2/L3 test layout, Buildkite steps, and test helpers (#2556)

Signed-off-by: wangyu <410167048@qq.com>
Signed-off-by: Huang, Zeyu <11222265+fhfuih@users.noreply.github.com>
Signed-off-by: wangyu <53896905+yenuo26@users.noreply.github.com>
Co-authored-by: Huang, Zeyu <11222265+fhfuih@users.noreply.github.com>

tests/model_executor/conftest.py

@@ -0,0 +1,10 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM-Omni project
+"""Reserved for model_executor-only hooks.
+
+vLLM custom-op bootstrap runs in the root :file:`tests/conftest.py` (at import
+time, before ``pytest_plugins``) so it always precedes any test module imports.
+A subdirectory :func:`pytest_configure` hook can be too late when pytest
+collects other packages first, which re-triggers
+``vllm::flashinfer_rotary_embedding``-style duplicate registrations.
+"""