bpo-40280: Skip socket, fork, subprocess tests on Emscripten (GH-31986)

- Add requires_fork and requires_subprocess to more tests
- Skip extension import tests if dlopen is not available
- Don't assume that _testcapi is a shared extension
- Skip a lot of socket tests that don't work on Emscripten
- Skip mmap tests, mmap emulation is incomplete
- venv does not work yet
- Cannot get libc from executable

The "entire" test suite is now passing on Emscripten with EMSDK from git head (91 suites are skipped).

1 files changed, 3 insertions, 0 deletions

diff --git a/Lib/unittest/test/testmock/testasync.py b/Lib/unittest/test/testmock/testasync.py
index 122e695663..1bab671acd 100644
--- a/Lib/unittest/test/testmock/testasync.py
+++ b/Lib/unittest/test/testmock/testasync.py
@@ -4,6 +4,9 @@ import inspect
 import re
 import unittest
 from contextlib import contextmanager
+from test import support
+
+support.requires_working_socket(module=True)
 
 from asyncio import run, iscoroutinefunction
 from unittest import IsolatedAsyncioTestCase