chore: Convert direct multiprocessing.set_start_method("forkserver") call to a pytest fixture. (#4377)

* chore: convert multiprocessing set_spawn to fixture in pytest

* Switch to early return

Author: Skylion007

tests/conftest.py

@@ -17,7 +17,12 @@
 # Early diagnostic for failed imports
 import pybind11_tests
 
-if os.name != "nt":
+
+@pytest.fixture(scope="session", autouse=True)
+def always_forkserver_on_unix():
+    if os.name == "nt":
+        return
+
     # Full background: https://github.com/pybind/pybind11/issues/4105#issuecomment-1301004592
     # In a nutshell: fork() after starting threads == flakiness in the form of deadlocks.
     # It is actually a well-known pitfall, unfortunately without guard rails.
@@ -27,6 +32,7 @@
     # running with defaults.
     multiprocessing.set_start_method("forkserver")
 
+
 _long_marker = re.compile(r"([0-9])L")
 _hexadecimal = re.compile(r"0x[0-9a-fA-F]+")