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_telnetlib.py

@@ -8,6 +8,8 @@ from test import support
 from test.support import socket_helper
 import unittest
 
+support.requires_working_socket(module=True)
+
 HOST = socket_helper.HOST
 
 def server(evt, serv):