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_importlib/extension/test_loader.py

@@ -12,11 +12,14 @@ import importlib.util
 import importlib
 from test.support.script_helper import assert_python_failure
 
+
 class LoaderTests(abc.LoaderTests):
 
     """Test load_module() for extension modules."""
 
     def setUp(self):
+        if not self.machinery.EXTENSION_SUFFIXES:
+            raise unittest.SkipTest("Requires dynamic loading support.")
         self.loader = self.machinery.ExtensionFileLoader(util.EXTENSIONS.name,
                                                          util.EXTENSIONS.file_path)
 
@@ -91,6 +94,8 @@ class MultiPhaseExtensionModuleTests(abc.LoaderTests):
     # Test loading extension modules with multi-phase initialization (PEP 489).
 
     def setUp(self):
+        if not self.machinery.EXTENSION_SUFFIXES:
+            raise unittest.SkipTest("Requires dynamic loading support.")
         self.name = '_testmultiphase'
         finder = self.machinery.FileFinder(None)
         self.spec = importlib.util.find_spec(self.name)