test(trash): count a volume that unmounted after a busy detach as detached (#499)

On macos-15-intel a detach that reported "Resource busy" finished afterwards, so
the retry found no such volume (portability run 40957282805). The loop now checks
os.path.ismount before each attempt.

tests/test_trash_macos_native.py

@@ -78,6 +78,8 @@ def _detach(mount):
         return f"this test still held files open under {mount}: {ours}"
     deadline = time.monotonic() + DETACH_WAIT_SECONDS
     while True:
+        if not os.path.ismount(mount):
+            return None  # a "busy" detach can still finish after it returns
         done = _hdiutil("detach", mount)
         if done.returncode == 0:
             return None