bpo-40280: Skip more tests/features that don't apply to Emscripten (GH-31791)

- fd inheritance can't be modified because Emscripten doesn't support subprocesses anyway.
- setpriority always fails
- geteuid no longer causes problems with latest emsdk
- umask is a stub
- geteuid / getuid always return 0, but process cannot chown to random uid.

Lib/test/test_posix.py

@@ -30,8 +30,11 @@ except ImportError:
 _DUMMY_SYMLINK = os.path.join(tempfile.gettempdir(),
                               os_helper.TESTFN + '-dummy-symlink')
 
-requires_32b = unittest.skipUnless(sys.maxsize < 2**32,
-        'test is only meaningful on 32-bit builds')
+requires_32b = unittest.skipUnless(
+    # Emscripten has 32 bits pointers, but support 64 bits syscall args.
+    sys.maxsize < 2**32 and not support.is_emscripten,
+    'test is only meaningful on 32-bit builds'
+)
 
 def _supports_sched():
     if not hasattr(posix, 'sched_getscheduler'):
@@ -578,6 +581,7 @@ class PosixTester(unittest.TestCase):
 
     @unittest.skipUnless(hasattr(os, 'O_CLOEXEC'), "needs os.O_CLOEXEC")
     @support.requires_linux_version(2, 6, 23)
+    @support.requires_subprocess()
     def test_oscloexec(self):
         fd = os.open(os_helper.TESTFN, os.O_RDONLY|os.O_CLOEXEC)
         self.addCleanup(os.close, fd)
@@ -737,7 +741,11 @@ class PosixTester(unittest.TestCase):
             is_root = (uid in (0, 1))
         else:
             is_root = (uid == 0)
-        if is_root:
+        if support.is_emscripten:
+            # Emscripten getuid() / geteuid() always return 0 (root), but
+            # cannot chown uid/gid to random value.
+            pass
+        elif is_root:
             # Try an amusingly large uid/gid to make sure we handle
             # large unsigned values.  (chown lets you use any
             # uid/gid you like, even if they aren't defined.)