Issue #15747: skip chflags UF_IMMUTABLE tests if EOPNOTSUPP is raised.

This is necessary for ZFS systems, which don't support UF_IMMUTABLE.

(Note: this commit is a manual merge of 78699:019a2390b014 as both
_test_chflags_regular_file and test_lchflags_symlink differ between
3.2 and default.)