Fix `is_forking_spawner` fixture to call helper fn

(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

(cherry picked from commit 83b6a3373a)

tractor/_testing/pytest.py

@@ -668,6 +668,7 @@ def pytest_generate_tests(
     #         scope='module',
     #     )
 
+
 def _is_forking_spawner(
     start_method: str,
 ) -> bool:
@@ -685,7 +686,7 @@ def is_forking_spawner(
     Is the `pytest` run using a `fork()`ing process spawning-backend?
 
     '''
-    return _is_forking_spawner
+    return _is_forking_spawner(start_method)
 
 
 def maybe_xfail_for_spawner(