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/pythoninfo.py

@@ -6,6 +6,7 @@ import errno
 import re
 import sys
 import traceback
+import unittest
 import warnings
 
 
@@ -615,7 +616,7 @@ def collect_resource(info_add):
 def collect_test_socket(info_add):
     try:
         from test import test_socket
-    except ImportError:
+    except (ImportError, unittest.SkipTest):
         return
 
     # all check attributes like HAVE_SOCKET_CAN