chore(llmobs): assert using span events instead of mocks (#11856)

Follow up of #11781 to further clean up LLMObs tests, specifically
replacing potentially flaky LLMObs span writer mocks and assertions with
the test LLMObsSpanWriter dummy class. Also clean up the
`tests/llmobs/conftest.py` file which previously contained a ton of
rarely used and sometimes redundant fixtures.

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

tests/llmobs/conftest.py

@@ -31,16 +31,6 @@ def pytest_configure(config):
     config.addinivalue_line("markers", "vcr_logs: mark test to use recorded request/responses")
 
 
-@pytest.fixture
-def mock_llmobs_span_writer():
-    patcher = mock.patch("ddtrace.llmobs._llmobs.LLMObsSpanWriter")
-    LLMObsSpanWriterMock = patcher.start()
-    m = mock.MagicMock()
-    LLMObsSpanWriterMock.return_value = m
-    yield m
-    patcher.stop()
-
-
 @pytest.fixture
 def mock_llmobs_eval_metric_writer():
     patcher = mock.patch("ddtrace.llmobs._llmobs.LLMObsEvalMetricWriter")
@@ -127,44 +117,6 @@ def default_global_config():
     return {"_dd_api_key": "<not-a-real-api_key>", "_llmobs_ml_app": "unnamed-ml-app"}
 
 
-@pytest.fixture
-def LLMObs(
-    mock_llmobs_span_writer, mock_llmobs_eval_metric_writer, mock_llmobs_evaluator_runner, ddtrace_global_config
-):
-    global_config = default_global_config()
-    global_config.update(ddtrace_global_config)
-    with override_global_config(global_config):
-        dummy_tracer = DummyTracer()
-        llmobs_service.enable(_tracer=dummy_tracer)
-        yield llmobs_service
-        llmobs_service.disable()
-
-
-@pytest.fixture
-def AgentlessLLMObs(
-    mock_llmobs_span_writer,
-    mock_llmobs_eval_metric_writer,
-    mock_llmobs_evaluator_runner,
-    ddtrace_global_config,
-):
-    global_config = default_global_config()
-    global_config.update(ddtrace_global_config)
-    global_config.update(dict(_llmobs_agentless_enabled=True))
-    with override_global_config(global_config):
-        dummy_tracer = DummyTracer()
-        llmobs_service.enable(_tracer=dummy_tracer)
-        yield llmobs_service
-        llmobs_service.disable()
-
-
-@pytest.fixture
-def disabled_llmobs():
-    prev = llmobs_service.enabled
-    llmobs_service.enabled = False
-    yield
-    llmobs_service.enabled = prev
-
-
 @pytest.fixture
 def mock_ragas_dependencies_not_present():
     import ragas
@@ -177,7 +129,7 @@ def mock_ragas_dependencies_not_present():
 
 
 @pytest.fixture
-def ragas(mock_llmobs_span_writer, mock_llmobs_eval_metric_writer):
+def ragas(mock_llmobs_eval_metric_writer):
     with override_global_config(dict(_dd_api_key="<not-a-real-key>")):
         try:
             import ragas