Site Health: Ensure that the tests returned by `WP_Site_Health::get_tests()` always have the required array keys: `direct` and `async`.

This avoids PHP notices if these keys were accidentally removed using the `site_status_tests` filter.

Props khag7, Ov3rfly, desrosj, Clorith.
Merges [48808] to the 5.5 branch.
Fixes #50858.

git-svn-id: https://develop.svn.wordpress.org/branches/5.5@48810 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/class-wp-site-health.php

@@ -2207,6 +2207,15 @@ class WP_Site_Health {
 		 */
 		$tests = apply_filters( 'site_status_tests', $tests );
 
+		// Ensure that the filtered tests contain the required array keys.
+		$tests = array_merge(
+			array(
+				'direct' => array(),
+				'async'  => array(),
+			),
+			$tests
+		);
+
 		return $tests;
 	}