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

If preadv2 is not available, preadv will raise NotImplementedError.
diff --git a/Lib/test/test_posix.py b/Lib/test/test_posix.py
index 5c93d0d..0f07a8f 100644
--- a/Lib/test/test_posix.py
+++ b/Lib/test/test_posix.py
@@ -310,6 +310,8 @@
             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