fails:IO::Buffer#external? with a file-backed buffer created with .map is true for a regular mapping
fails:IO::Buffer#external? with a file-backed buffer created with .map is false for a private mapping
fails:IO::Buffer#external? with a String-backed buffer created with .for is true for a buffer created without a block
fails:IO::Buffer#external? with a String-backed buffer created with .for is true for a buffer created with a block
fails:IO::Buffer#external? with a String-backed buffer created with .string is true
fails:IO::Buffer#external? with a slice of a buffer created with .map is false
fails:IO::Buffer#external? is true for a buffer with externally-managed memory
