bpo-40280: Skip socket, fork, subprocess tests on Emscripten

[tiran]

• 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).

https://bugs.python.org/issue40280

Automerge-Triggered-By: GH:tiran

[tiran]

== Tests result: SUCCESS ==

341 tests OK.

91 tests skipped:
    test__xxsubinterpreters test_asyncgen test_asynchat test_asyncio
    test_asyncore test_check_c_globals test_clinic test_cmd_line
    test_concurrent_futures test_contextlib_async test_ctypes
    test_curses test_dbm_gnu test_dbm_ndbm test_devpoll test_doctest
    test_docxmlrpc test_embed test_epoll test_faulthandler test_fcntl
    test_file_eintr test_fork1 test_ftplib test_gdb test_grp
    test_httplib test_httpservers test_idle test_imaplib
    test_interpreters test_ioctl test_kqueue test_lzma test_mmap
    test_msilib test_multiprocessing_fork
    test_multiprocessing_forkserver test_multiprocessing_main_handling
    test_multiprocessing_spawn test_nis test_openpty test_ossaudiodev
    test_pdb test_poll test_poplib test_pty test_pwd test_readline
    test_regrtest test_repl test_select test_selectors test_smtplib
    test_smtpnet test_socket test_socketserver test_spwd test_sqlite3
    test_ssl test_stable_abi_ctypes test_startfile test_subprocess
    test_sys_settrace test_syslog test_tcl test_telnetlib test_tix
    test_tk test_tools test_ttk_guionly test_ttk_textonly test_turtle
    test_urllib2 test_urllib2_localnet test_urllib2net test_urllibnet
    test_venv test_wait3 test_wait4 test_webbrowser test_winconsoleio
    test_winreg test_winsound test_wsgiref test_xmlrpc test_xmlrpc_net
    test_xxlimited test_zipfile64 test_zipimport_support test_zoneinfo

1 test run no tests:
    test_dtrace

Total duration: 11 min 13 sec
Tests result: SUCCESS