gh-90473: WASI: skip gethostname tests (GH-93092)

- WASI's ``gethostname()`` is a stub that always fails with OSError
  ``ENOTSUP``
- skip mailcap ``test`` if subprocess is not available
- WASI process_time clock does not work.
(cherry picked from commit 760ec8940a)

Co-authored-by: Christian Heimes <christian@python.org>

Lib/test/test_mailbox.py

@@ -10,12 +10,17 @@ import io
 import tempfile
 from test import support
 from test.support import os_helper
+from test.support import socket_helper
 import unittest
 import textwrap
 import mailbox
 import glob
 
 
+if not socket_helper.has_gethostname:
+    raise unittest.SkipTest("test requires gethostname()")
+
+
 class TestBase:
 
     all_mailbox_types = (mailbox.Message, mailbox.MaildirMessage,