Fix test_posix if RWF_HIPRI is defined but not preadv2. (GH-13980)

If preadv2 is not available, preadv will raise NotImplementedError.
(cherry picked from commit 44867bb)

Co-authored-by: Benjamin Peterson <benjamin@python.org>

Lib/test/test_posix.py

@@ -308,6 +308,8 @@ def test_preadv_flags(self):
             buf = [bytearray(i) for i in [5, 3, 2]]
             self.assertEqual(posix.preadv(fd, buf, 3, os.RWF_HIPRI), 10)
             self.assertEqual([b't1tt2', b't3t', b'5t'], list(buf))
+        except NotImplementedError:
+            self.skipTest("preadv2 not available")
         except OSError as inst:
             # Is possible that the macro RWF_HIPRI was defined at compilation time
             # but the option is not supported by the kernel or the runtime libc shared