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).

setup.py

@@ -84,10 +84,15 @@ CYGWIN = (HOST_PLATFORM == 'cygwin')
 MACOS = (HOST_PLATFORM == 'darwin')
 AIX = (HOST_PLATFORM.startswith('aix'))
 VXWORKS = ('vxworks' in HOST_PLATFORM)
+EMSCRIPTEN = HOST_PLATFORM == 'emscripten-wasm32'
 CC = os.environ.get("CC")
 if not CC:
     CC = sysconfig.get_config_var("CC")
 
+if EMSCRIPTEN:
+    # emcc is a Python script from a different Python interpreter.
+    os.environ.pop("PYTHONPATH", None)
+
 
 SUMMARY = """
 Python is an interpreted, interactive, object-oriented programming