Check if we're on the main thread when using mock sigterm handler in …

…tests (#11552)

tests/public/flows/test_flow_crashes.py

@@ -9,6 +9,7 @@
 import asyncio
 import os
 import signal
+import threading
 from unittest.mock import ANY, MagicMock
 
 import anyio
@@ -211,6 +212,8 @@ async def parent_flow():
 
 @pytest.fixture
 def mock_sigterm_handler():
+    if threading.current_thread() != threading.main_thread():
+        pytest.skip("Can't test signal handlers from a thread")
     mock = MagicMock()
 
     def handler(*args, **kwargs):

tests/test_flows.py

@@ -5,6 +5,7 @@
 import os
 import signal
 import sys
+import threading
 import time
 from functools import partial
 from itertools import combinations
@@ -82,6 +83,8 @@ def test_flow():
 
 @pytest.fixture
 def mock_sigterm_handler():
+    if threading.current_thread() != threading.main_thread():
+        pytest.skip("Can't test signal handlers from a thread")
     mock = MagicMock()
 
     def handler(*args, **kwargs):