Observers add an extra tmp, breaking the expected output.

This test still fails on i386 Windows with "Allowed memory size of %d bytes
exhausted" because the output buffer grows too big. My first intuition was to
add a chunk_size to ob_start, but this won't work if the output buffer is
flushed deep into the call stack, causing a premature or just a second stack
limit error. So, reduce the stack size in an attempt to produce less output.

* PHP-8.4:
  [skip ci] Reduce zend.max_allowed_stack_size in gh20840.phpt

* PHP-8.5:
  [skip ci] Reduce zend.max_allowed_stack_size in gh20840.phpt

The computed number of properties using zend_hash_num_elements(zobj->properties)
is incorrect when the object contains virtual properties. We don't have a
trivial way to find the number of properties virtual properties that need to be
added to this number, so just append with zend_hash_add_new() instead.

Fixes GH-20479
Closes GH-20988

* PHP-8.4:
  Fix hooked object properties overflow

* PHP-8.5:
  Fix hooked object properties overflow