Fix GH-16414: zend_test.observer.observe_function_names may segfault

Unless `zend_test.observer.enabled` is on, we must not add observer
handlers, so we let the INI modify handler fail e

Fix GH-16414: zend_test.observer.observe_function_names may segfault

Unless `zend_test.observer.enabled` is on, we must not add observer
handlers, so we let the INI modify handler fail early.

We also need to ensure that the functions to observe have already been
called, so that their begin and end handlers are properly initialized.
Otherwise we will not observe the function execution, but a segfault.

Co-authored-by: Bob Weinand <bobwei9@hotmail.com>

Closes GH-16438.

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