Illumos doesn't seem to mind self-sendfile but doesn't corrupt the
file; make the test a little more forgiving.

--- Python-3.12.1/Lib/test/test_shutil.py.~1~	Thu Dec  7 12:45:44 2023
+++ Python-3.12.1/Lib/test/test_shutil.py	Wed Feb  7 10:22:17 2024
@@ -2874,8 +2874,13 @@
     def test_same_file(self):
         self.addCleanup(self.reset)
         with self.get_files() as (src, dst):
-            with self.assertRaises((OSError, _GiveupOnFastCopy)):
+            try:
                 self.zerocopy_fun(src, src)
+            except OSError:
+                pass
+            except _GiveupOn_FastCopy:
+                pass
+
         # Make sure src file is not corrupted.
         self.assertEqual(read_file(TESTFN, binary=True), self.FILEDATA)