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/unittest/test/test_runner.py

@@ -3,6 +3,7 @@ import os
 import sys
 import pickle
 import subprocess
+from test import support
 
 import unittest
 from unittest.case import _Outcome
@@ -1139,6 +1140,7 @@ class Test_TextTestRunner(unittest.TestCase):
         expectedresult = (runner.stream, DESCRIPTIONS, VERBOSITY)
         self.assertEqual(runner._makeResult(), expectedresult)
 
+    @support.requires_subprocess()
     def test_warnings(self):
         """
         Check that warnings argument of TextTestRunner correctly affects the