Add directories for tests relating to
- `break` statements
- functions with by-ref returns
- `class_exists()`
- dynamic properties
- error handlers
- error suppression with `@`
- `get_object_vars()`
- `highlight_string()` and `highlight_file()`
- interfaces
- `is_callable()`
- iterators
- shutdown functions
- streams
As well as organizing a couple of tests into existing sub directories along the
way
Work towards GH-15631
Daniel Scherzer
886ee6d981