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_pydoc.py

@@ -27,7 +27,7 @@ from test.support import os_helper
 from test.support.script_helper import assert_python_ok, assert_python_failure
 from test.support import threading_helper
 from test.support import (reap_children, captured_output, captured_stdout,
-                          captured_stderr, requires_docstrings)
+                          captured_stderr, is_emscripten, requires_docstrings)
 from test.support.os_helper import (TESTFN, rmtree, unlink)
 from test import pydoc_mod
 
@@ -1339,6 +1339,7 @@ foo
         )
 
 
+@unittest.skipIf(is_emscripten, "Socket server not available on Emscripten.")
 class PydocServerTest(unittest.TestCase):
     """Tests for pydoc._start_server"""