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_asyncio/__init__.py

@@ -1,7 +1,9 @@
 import os
+from test import support
 from test.support import load_package_tests
 from test.support import import_helper
 
+support.requires_working_socket(module=True)
 
 # Skip tests if we don't have concurrent.futures.
 import_helper.import_module('concurrent.futures')