[feature/extension-manager] Remove cron's dependency on the extension manager.

Instead a separate cron provider supplies the manager with tasks from the
extension finder.

PHPBB3-10323

tests/cron/tasks/simple_not_runnable.php

@@ -0,0 +1,13 @@
+<?php
+
+class phpbb_cron_task_core_simple_not_runnable extends phpbb_cron_task_base
+{
+	public function run()
+	{
+	}
+
+	public function is_runnable()
+	{
+		return false;
+	}
+}

tests/cron/tasks/simple_ready.php

@@ -0,0 +1,8 @@
+<?php
+
+class phpbb_cron_task_core_simple_ready extends phpbb_cron_task_base
+{
+	public function run()
+	{
+	}
+}

tests/cron/tasks/simple_should_not_run.php

@@ -0,0 +1,13 @@
+<?php
+
+class phpbb_cron_task_core_simple_should_not_run extends phpbb_cron_task_base
+{
+	public function run()
+	{
+	}
+
+	public function should_run()
+	{
+		return false;
+	}
+}