Hooks: Fix resort_active_iterations() skipping next priority on self-removal

[mrcasual]

Trac ticket: https://core.trac.wordpress.org/ticket/64653

When a callback removes itself during do_action() / apply_filters() and is the sole callback at its priority, resort_active_iterations() positions the internal array pointer one step too far. The main loop's next() call then skips the following priority entirely.

Steps to reproduce

<?php

add_action( 'test_hook', '__return_true', 10 );

add_action( 'test_hook', function () {
    remove_action( 'test_hook', __FUNCTION__, 50 );
}, 50 );

add_action( 'test_hook', function () {
    error_log( 'This never runs' );
}, 100 );

do_action( 'test_hook' ); // Priority 100 is silently skipped.

The bug requires: (1) a lower priority exists, (2) the removed callback is the only one at its priority.

[github-actions]

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[github-actions]

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[westonruter]

I can confirm this test fails without the fix applied:

1) Tests_Hooks_DoAction::test_self_removing_callback_does_not_skip_next_priority
Priority 100 should not be skipped when priority 50 removes itself during iteration.
Failed asserting that two arrays are identical.
--- Expected
+++ Actual
@@ @@
 Array &0 (
     0 => 10
     1 => 50
-    2 => 100
 )

/var/www/tests/phpunit/tests/hooks/doAction.php:331

[westonruter]

This test does not fail without the fix applied.