gh-109981: Resolve situation on iOS regarding fd_count. (#132823)

Modifies the test helper that counts the list of open file descriptors to use
the optimised ``/dev/fd`` approach on all Apple platforms, not just macOS. This
avoids crashes caused by guarded file descriptors.

Author: johnzhou721

Lib/test/support/os_helper.py

@@ -657,7 +657,7 @@ def fd_count():
     """
     if sys.platform.startswith(('linux', 'android', 'freebsd', 'emscripten')):
         fd_path = "/proc/self/fd"
-    elif sys.platform == "darwin":
+    elif support.is_apple:
         fd_path = "/dev/fd"
     else:
         fd_path = None

Misc/NEWS.d/next/Tests/2025-04-23-02-23-37.gh-issue-109981.IX3k8p.rst

@@ -0,0 +1,3 @@
+The test helper that counts the list of open file descriptors now uses the
+optimised ``/dev/fd`` approach on all Apple platforms, not just macOS.
+This avoids crashes caused by guarded file descriptors.