bpo-40280: Block more non-working syscalls in Emscripten (GH-31757)

- getgroups always fails.
- geteuid and getegid always return 0 (root), which confuse tarfile and
  tests.
- hardlinks (link, linkat) always fails.
- non-encodable file names are not supported by NODERAWFS layer.
- mark more tests with dependency on subprocess and multiprocessing.
  Mocking does not work if the module fails to import.

Lib/test/support/os_helper.py

@@ -49,8 +49,8 @@ if os.name == 'nt':
                   'encoding (%s). Unicode filename tests may not be effective'
                   % (TESTFN_UNENCODABLE, sys.getfilesystemencoding()))
             TESTFN_UNENCODABLE = None
-# Mac OS X denies unencodable filenames (invalid utf-8)
-elif sys.platform != 'darwin':
+# macOS and Emscripten deny unencodable filenames (invalid utf-8)
+elif sys.platform not in {'darwin', 'emscripten'}:
     try:
         # ascii and utf-8 cannot encode the byte 0xff
         b'\xff'.decode(sys.getfilesystemencoding())