[PromptFlow][TestCase] Fix test case to enable them passable in diffe…

…rent systems. (#1038)

# Description
To enable test cases to work on different systems(win, mac, linux), set
the process start method to spawn.


# All Promptflow Contribution checklist:
- [ ] **The pull request does not introduce [breaking changes].**
- [ ] **CHANGELOG is updated for new features, bug fixes or other
significant changes.**
- [ ] **I have read the [contribution guidelines](../CONTRIBUTING.md).**
- [ ] **Create an issue and link to the pull request to get dedicated
review from promptflow team. Learn more: [suggested
workflow](../CONTRIBUTING.md#suggested-workflow).**

## General Guidelines and Best Practices
- [ ] Title of the pull request is clear and informative.
- [ ] There are a small number of commits, each of which have an
informative message. This means that previously merged commits do not
appear in the history of the PR. For more information on cleaning up the
commits in your PR, [see this
page](https://github.com/Azure/azure-powershell/blob/master/documentation/development-docs/cleaning-up-commits.md).

### Testing Guidelines
- [ ] Pull request includes test coverage for the included changes.

src/promptflow/tests/executor/unittests/processpool/test_healthy_ensured_process.py

@@ -27,7 +27,7 @@ def end_process(healthy_ensured_process):
 class TestHealthyEnsuredProcess:
 
     def test_healthy_ensured_process(self):
-        context = get_multiprocessing_context("fork")
+        context = get_multiprocessing_context("spawn")
         healthy_ensured_process = HealthyEnsuredProcess(executor_creation_func, context)
         assert healthy_ensured_process.is_ready is False
         task_queue = Queue()
@@ -38,7 +38,7 @@ def test_healthy_ensured_process(self):
         assert healthy_ensured_process.process.is_alive() is False
 
     def test_unhealthy_process(self):
-        context = get_multiprocessing_context("fork")
+        context = get_multiprocessing_context("spawn")
         healthy_ensured_process = HealthyEnsuredProcess(executor_creation_func_timeout, context)
         assert healthy_ensured_process.is_ready is False
         task_queue = Queue()
@@ -49,7 +49,7 @@ def test_unhealthy_process(self):
         assert healthy_ensured_process.process.is_alive() is False
 
     def test_format_current_process(self):
-        context = get_multiprocessing_context("fork")
+        context = get_multiprocessing_context("spawn")
         healthy_ensured_process = HealthyEnsuredProcess(executor_creation_func, context)
         healthy_ensured_process.process = patch(
             'promptflow.executor._line_execution_process_pool.Process', autospec=True)
@@ -66,7 +66,7 @@ def test_format_current_process(self):
 
     @patch('promptflow.executor._line_execution_process_pool.logger.info', autospec=True)
     def test_format_completed_process(self, mock_logger_info):
-        context = get_multiprocessing_context("fork")
+        context = get_multiprocessing_context("spawn")
         healthy_ensured_process = HealthyEnsuredProcess(executor_creation_func, context)
         healthy_ensured_process.process = patch(
             'promptflow.executor._line_execution_process_pool.Process', autospec=True)