gh-90473: Skip and document more failing tests on WASI (GH-93436)

- Mark more ``umask()`` cases
- ``dup()`` is not supported
- ``/dev/null`` is not available
- document missing features
- mark more modules as not available

Lib/test/test_os.py

@@ -186,6 +186,9 @@ class FileTests(unittest.TestCase):
     @unittest.skipIf(
         support.is_emscripten, "Test is unstable under Emscripten."
     )
+    @unittest.skipIf(
+        support.is_wasi, "WASI does not support dup."
+    )
     def test_closerange(self):
         first = os.open(os_helper.TESTFN, os.O_CREAT|os.O_RDWR)
         # We must allocate two consecutive file descriptors, otherwise
@@ -1588,7 +1591,10 @@ class MakedirTests(unittest.TestCase):
                             'dir5', 'dir6')
         os.makedirs(path)
 
-    @unittest.skipIf(support.is_emscripten, "Emscripten's umask is a stub.")
+    @unittest.skipIf(
+        support.is_emscripten or support.is_wasi,
+        "Emscripten's/WASI's umask is a stub."
+    )
     def test_mode(self):
         with os_helper.temp_umask(0o002):
             base = os_helper.TESTFN