feat: D69478008 [llama-stack] turning tests into data-driven (#1180)

# What does this PR do?

We have several places running tests for different purposes.
- oss llama stack
  - provider tests
  - e2e tests
- provider llama stack
  - unit tests
  - e2e tests

It would be nice if they can *share the same set of test data*, so we
maintain the consistency between spec and implementation. This is what
this diff is about, isolating test data from test coding, so that we can
reuse the same data at different places by writing different test
coding.

## Test Plan

== Set up Ollama local server  
==  Run a provider test
conda activate stack

OLLAMA_URL="http://localhost:8321" \
pytest -v -s -k "ollama" --inference-model="llama3.2:3b-instruct-fp16" \

llama_stack/providers/tests/inference/test_text_inference.py::TestInference::test_completion_structured_output
// test_structured_output should also work

== Run an e2e test
conda activate sherpa
with-proxy pip install llama-stack
export INFERENCE_MODEL=llama3.2:3b-instruct-fp16
export LLAMA_STACK_PORT=8322
with-proxy llama stack build --template ollama
with-proxy llama stack run --env OLLAMA_URL=http://localhost:8321 ollama
  - Run test client,
LLAMA_STACK_PORT=8322 LLAMA_STACK_BASE_URL="http://localhost:8322" \
pytest -v -s --inference-model="llama3.2:3b-instruct-fp16" \

tests/client-sdk/inference/test_text_inference.py::test_text_completion_structured_output
// test_text_chat_completion_structured_output should also work

## Notes

- This PR was automatically generated by oss_sync
- Please refer to D69478008 for more details.

llama_stack/providers/tests/README.md

@@ -104,3 +104,6 @@ pytest llama_stack/providers/tests/ --config=ci_test_config.yaml
 Currently, we support test config on inference, agents and memory api tests.
 
 Example format of test config can be found in ci_test_config.yaml.
+
+## Test Data
+We encourage providers to use our test data for internal development testing, so to make it easier and consistent with the tests we provide. Each test case may define its own data format, and please refer to our test source code to get details on how these fields are used in the test.