MDL-79675 enrol_lti: fix mocks depending on previously untyped props

When $launch_id wasn't typed, it was ok to call the getter, but now
that it is, php throws an error. Replaced with stub method.

enrol/lti/tests/local/ltiadvantage/lti_advantage_testcase.php

@@ -138,7 +138,7 @@ protected function get_mock_launch(\stdClass $resource, array $mockuser,
             ?array $customparams = null, $aud = '123'): LtiMessageLaunch {
 
         $mocklaunch = $this->getMockBuilder(LtiMessageLaunch::class)
-            ->onlyMethods(['getLaunchData'])
+            ->onlyMethods(['getLaunchData', 'getLaunchId'])
             ->disableOriginalConstructor()
             ->getMock();
         $mocklaunch->expects($this->any())
@@ -246,6 +246,12 @@ function()
                 }
             ));
 
+        $mocklaunch->expects($this->any())
+            ->method('getLaunchId')
+            ->will($this->returnCallback(function() {
+                return uniqid('lti1p3_launch_', true);
+            }));
+
         return $mocklaunch;
     }