Fix `tractor_test` kwarg check and Windows `start_method` default

- Use `kw in kwargs` membership test instead of
  `kwargs[kw]` to avoid `KeyError` on missing params.
- Restructure Windows `start_method` logic to properly
  default to `'trio'` when unset; only raise on an
  explicit non-trio value.

Review: PR #427 (Copilot)
https://github.com/goodboy/tractor/pull/427#pullrequestreview-4009934142

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

tractor/_testing/pytest.py

@@ -118,18 +118,13 @@ def tractor_test(
             'timeout',
         ]:
             if kw in inspect.signature(wrapped).parameters:
-                assert kwargs[kw]
+                assert kw in kwargs
 
-        if (
+        start_method = kwargs.get('start_method')
-            (start_method := kwargs.get('start_method'))
+        if platform.system() == "Windows":
-            is
+            if start_method is None:
-            None
+                kwargs['start_method'] = 'trio'
-        ):
+            elif start_method != 'trio':
-            if (
-                platform.system() == "Windows"
-                and
-                start_method != 'trio'
-            ):
                 raise ValueError(
                     'ONLY the `start_method="trio"` is supported on Windows.'
                 )