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, 2 insertions, 2 deletions

diff --git a/Lib/test/test_wait3.py b/Lib/test/test_wait3.py
index aa166ba..4ec7690 100644
--- a/Lib/test/test_wait3.py
+++ b/Lib/test/test_wait3.py
@@ -9,8 +9,8 @@ import unittest
 from test.fork_wait import ForkWait
 from test import support
 
-if not hasattr(os, 'fork'):
-    raise unittest.SkipTest("os.fork not defined")
+if not support.has_fork_support:
+    raise unittest.SkipTest("requires working os.fork()")
 
 if not hasattr(os, 'wait3'):
     raise unittest.SkipTest("os.wait3 not defined")