SyncMadeEasy/force-app/main/default/classes/CustomSettingInsertion_Test.cls

@isTest
public class CustomSettingInsertion_Test {

    @istest public static void Custsetting ()
    {
        Test.startTest();
        CustomSettingInsertion.onInstall();
        Test.stopTest(); 
        System.assert(true);
    }

}