Issue #3246673: Move PendingUpdatesValidator into Package Manager

package_manager/tests/fixtures/db_update.php

@@ -6,7 +6,9 @@
  */
 
 /**
- * Here is a fake update.
+ * Here is a fake update hook.
+ *
+ * The schema version is the maximum possible value for a 32-bit integer.
  */
-function package_manager_update_50000() {
+function package_manager_update_2147483647() {
 }