MDL-39846 use new special value -1 in $CFG->loglifetime to prevent legacy logging

lib/classes/event/base.php

@@ -459,7 +459,7 @@ abstract class base implements \IteratorAggregate {
 
         $this->triggered = true;
 
-        if (!empty($CFG->loglifetime)) {
+        if (isset($CFG->loglifetime) and $CFG->loglifetime != -1) {
             if ($data = $this->get_legacy_logdata()) {
                 call_user_func_array('add_to_log', $data);
             }

lib/tests/event_test.php

@@ -29,6 +29,17 @@ require_once(__DIR__.'/fixtures/event_fixtures.php');
 
 class core_event_testcase extends advanced_testcase {
 
+    protected function setUp() {
+        global $CFG;
+        // No need to always modify log table here.
+        $CFG->loglifetime = '-1';
+    }
+
+    protected function tearDown() {
+        global $CFG;
+        $CFG->loglifetime = '0';
+    }
+
     public function test_event_properties() {
         global $USER;