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).

1 files changed, 4 insertions, 1 deletions

diff --git a/Lib/test/test_venv.py b/Lib/test/test_venv.py
index db812f2..c91b754 100644
--- a/Lib/test/test_venv.py
+++ b/Lib/test/test_venv.py
@@ -16,7 +16,7 @@ import sys
 import tempfile
 from test.support import (captured_stdout, captured_stderr, requires_zlib,
                           skip_if_broken_multiprocessing_synchronize, verbose,
-                          requires_subprocess)
+                          requires_subprocess, is_emscripten)
 from test.support.os_helper import (can_symlink, EnvironmentVarGuard, rmtree)
 import unittest
 import venv
@@ -34,6 +34,9 @@ requireVenvCreate = unittest.skipUnless(
     or sys._base_executable != sys.executable,
     'cannot run venv.create from within a venv on this platform')
 
+if is_emscripten:
+    raise unittest.SkipTest("venv is not available on Emscripten.")
+
 @requires_subprocess()
 def check_output(cmd, encoding=None):
     p = subprocess.Popen(cmd,