Add directories for tests relating to
- calling user functions (`call_user_func()` and `call_user_func_array()`)
- using `::class` to access class names
- null coalescing with `??`
- concatenation with `.`
- indirect function calls (e.g. by calling a variable with a function name)
- reporting of line numbers
- static variables in functions
- type casts
As well as organizing a couple of tests into existing sub directories along the
way
Work towards GH-15631
