Add root user checks to test.support

This partially backports commit 86b8617
(GH-146195); making existing tests use the helper is omitted.

Author: encukou

Lib/test/support/__init__.py

@@ -2598,3 +2598,8 @@ def control_characters_c0() -> list[str]:
     C0 control characters defined as the byte range 0x00-0x1F, and 0x7F.
     """
     return [chr(c) for c in range(0x00, 0x20)] + ["\x7F"]
+
+
+_ROOT_IN_POSIX = hasattr(os, 'geteuid') and os.geteuid() == 0
+requires_root_user = unittest.skipUnless(_ROOT_IN_POSIX, "test needs root privilege")
+requires_non_root_user = unittest.skipIf(_ROOT_IN_POSIX, "test needs non-root account")