Fix exception in Popen.__del__ in test on Windows

The newly introduced test would pass, but show:

    Exception ignored in: <function Popen.__del__ at 0x000002483DE14900>
    Traceback (most recent call last):
      File "C:\Program Files\WindowsApps\PythonSoftwareFoundation.Python.3.12_3.12.752.0_x64__qbz5n2kfra8p0\Lib\subprocess.py", line 1130, in __del__
        self._internal_poll(_deadstate=_maxsize)
      File "C:\Program Files\WindowsApps\PythonSoftwareFoundation.Python.3.12_3.12.752.0_x64__qbz5n2kfra8p0\Lib\subprocess.py", line 1575, in _internal_poll
        if _WaitForSingleObject(self._handle, 0) == _WAIT_OBJECT_0:
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    OSError: [WinError 6] The handle is invalid

This was due to how, at least for now, the deprecation warning
tests are not using GitPython's normal fixtures, which help clean
things up on Windows.

This adds a small amount of ad-hoc cleanup. It also moves the
acquisition of the Diff object into a pytest fixture, which can be
reused for the immediately forthcoming test that the preferred
property does not warn.

test/deprecation/test_various.py

@@ -3,18 +3,27 @@
 
 """Tests of assorted deprecation warnings with no extra subtleties to check."""
 
-from git.diff import NULL_TREE
-from git.repo import Repo
+import gc
 
 import pytest
 
+from git.diff import NULL_TREE
+from git.repo import Repo
+
 
-def test_diff_renamed_warns(tmp_path):
+@pytest.fixture
+def single_diff(tmp_path):
+    """Fixture to supply a single-file diff."""
     (tmp_path / "a.txt").write_text("hello\n", encoding="utf-8")
     repo = Repo.init(tmp_path)
     repo.index.add(["a.txt"])
     commit = repo.index.commit("Initial commit")
     (diff,) = commit.diff(NULL_TREE)  # Exactly one file in the diff.
+    yield diff
+    del repo, commit, diff
+    gc.collect()
+
 
+def test_diff_renamed_warns(single_diff):
     with pytest.deprecated_call():
-        diff.renamed
+        single_diff.renamed