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).

Lib/test/test_urllib2_localnet.py

@@ -8,6 +8,7 @@ import threading
 import unittest
 import hashlib
 
+from test import support
 from test.support import hashlib_helper
 from test.support import threading_helper
 from test.support import warnings_helper
@@ -17,6 +18,8 @@ try:
 except ImportError:
     ssl = None
 
+support.requires_working_socket(module=True)
+
 here = os.path.dirname(__file__)
 # Self-signed cert file for 'localhost'
 CERT_localhost = os.path.join(here, 'keycert.pem')