Skip test_preadv_flags if RWF_HIPRI is not supported by the system (GH-12762)

Lib/test/test_posix.py

@@ -310,6 +310,14 @@ class PosixTester(unittest.TestCase):
             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 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
+            # library.
+            if inst.errno in {errno.EINVAL, errno.ENOTSUP}:
+                raise unittest.SkipTest("RWF_HIPRI is not supported by the current system")
+            else:
+                raise
         finally:
             os.close(fd)