fails:IO::Buffer#shared? with a file-backed buffer created with .map is true for a regular mapping
fails:IO::Buffer#shared? with a file-backed buffer created with .map is false for a private mapping
fails:IO::Buffer#shared? with a slice of a buffer created with .map is false when slicing a regular file-backed buffer
fails:IO::Buffer#shared? with a slice of a buffer created with .map is false when slicing a private file-backed buffer
fails:IO::Buffer#shared? is true for a buffer created with SHARED flag
fails:IO::Buffer#shared? is true for a non-private buffer created with .map
