Remove now-unnecessary filterwarnings("ignore::ResourceWarning") decorators

The old tests had filterwarnings("ignore::ResourceWarning" if win32 ...)
because _terminate_process_tree didn't call process.wait() or close pipe
transports, so GC of the anyio Process wrapper emitted ResourceWarning.

_terminate_and_reap now explicitly calls wait() + closes stdin/stdout
pipes in the test's AsyncExitStack teardown (i.e., during the test,
before GC can fire). The warnings should no longer occur — if they do,
CI will tell us and we should fix the root cause rather than suppress.

Github-Issue: #1775

Author: maxisbey

tests/client/test_stdio.py

@@ -340,7 +340,6 @@ class TestChildProcessCleanup:
     """
 
     @pytest.mark.anyio
-    @pytest.mark.filterwarnings("ignore::ResourceWarning" if sys.platform == "win32" else "default")
     async def test_basic_child_process_cleanup(self):
         """Parent spawns one child; terminating the tree kills both."""
         async with AsyncExitStack() as stack:
@@ -364,7 +363,6 @@ async def test_basic_child_process_cleanup(self):
             await _assert_stream_closed(stream)
 
     @pytest.mark.anyio
-    @pytest.mark.filterwarnings("ignore::ResourceWarning" if sys.platform == "win32" else "default")
     async def test_nested_process_tree(self):
         """Parent → child → grandchild; terminating the tree kills all three."""
         async with AsyncExitStack() as stack:
@@ -401,7 +399,6 @@ async def test_nested_process_tree(self):
                 await _assert_stream_closed(stream)
 
     @pytest.mark.anyio
-    @pytest.mark.filterwarnings("ignore::ResourceWarning" if sys.platform == "win32" else "default")
     async def test_early_parent_exit(self):
         """Parent exits immediately on SIGTERM; process-group termination still
         catches the child (exercises the race where the parent dies mid-cleanup).