Sticky Posts logic is pretty broken

[WPprodigy]

Here, EP attempts to add a weight to sticky posts if on doing the homepage query: https://github.com/Automattic/ElasticPress/blob/develop/includes/classes/Indexable/Post/Post.php#L1188-L1217

This is how the core WP Query does it: https://github.com/WordPress/WordPress/blob/6b7ba33d6851fea58bc3214c2b9e4316aa918fa5/wp-includes/class-wp-query.php#L3131-L3133

I believe the checks in EP are not sufficient / errorsome, and should be more along these lines:

$sticky_posts  = get_option( 'sticky_posts' );
$on_first_page = ! isset( $args['paged'] ) || $args['paged'] <= 1;
$ignore_sticky = isset( $q['ignore_sticky_posts'] ) && $q['ignore_sticky_posts'];

// Check first if there's sticky posts and show them only in the front page.
if ( is_array( $sticky_posts ) && ! empty( $sticky_posts ) && is_home() && $on_first_page && ! $ignore_sticky ) {

Furthermore, these tests are broken / not actually testing anything: https://github.com/Automattic/ElasticPress/blob/develop/tests/php/indexables/TestPost.php#L4102-L4142. The second testStickyPostsExcludedOnNotHome test also falls into the same issue as #63 - it will sometimes fails if based on if the posts share a timestamp or not.

The tests are really meant to be testing the prioritization of sticky posts, but first need to of course send that test home query to ES. Something like this:

public function testStickyPostsPrioritizedOnHome() {
	// Setup three posts, with the sticky post chronologically in the middle.
	Functions\create_and_sync_post( array( 'post_title' => 'Post 1', 'post_date' => '2020-08-25 12:30:00' ) );
	$sticky_id = Functions\create_and_sync_post( array( 'post_title' => 'Post 2: Sticky', 'post_date' => '2020-08-25 12:31:00' ) );
	stick_post( $sticky_id );
	Functions\create_and_sync_post( array( 'post_title' => 'Post 3', 'post_date' => '2020-08-25 12:32:00' ) );
	ElasticPress\Elasticsearch::factory()->refresh_indices();

	add_filter( 'pre_get_posts', [ $this, 'send_home_query_to_es' ] );

	$this->go_to( '/' );
	$q = $GLOBALS['wp_query'];
	$this->assertEquals( 'Post 2: Sticky', $q->posts[0]->post_title );

	remove_filter( 'pre_get_posts', [ $this, 'send_home_query_to_es' ] );
}

Just making this an issue for now, as it's turned into a bit more of a timesink than intended. If anybody else wants to work on this, be sure to make sure the tests can actually fail if the special sticky posts logic is removed, rather than just relying on WP_Query core logic to carry the burden.