Zend/tests: organize some tests with subdirectories (#15638)

Move some low-hanging fruit, creating new directories for the tests for

* access modifiers
* `class_alias()`
* c

Zend/tests: organize some tests with subdirectories (#15638)

Move some low-hanging fruit, creating new directories for the tests for

* access modifiers
* `class_alias()`
* constant expressions
* constructor property promotion
* `__debugInfo()`
* dereferencing
* first class callable syntax

Additionally, move some tests into the existing subdirectory for
closure-related tests

Work towards GH-15631

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