refactor: set up tests/regressions

Signed-off-by: Travis Johnson <tsjohnso@us.ibm.com>

Author: tjohnson31415

.buildkite/test-pipeline.yaml

@@ -191,11 +191,10 @@ steps:
   mirror_hardwares: [amdexperimental, amdproduction]
   source_file_dependencies:
   - vllm/
-  - tests/test_regression
+  - tests/regressions
   commands:
   - pip install modelscope
-  - pytest -v -s test_regression.py
-  working_dir: "/vllm-workspace/tests" # optional
+  - pytest -v -s regressions
 
 - label: Engine Test # 10min
   mirror_hardwares: [amdexperimental, amdproduction]