Public class AutoConvertLeads
{
    @InvocableMethod
    public static void LeadAssign(List<Id> LeadIds)
    {
        system.debug('1');
        Database.LeadConvert leadConvert = new Database.LeadConvert();
        system.debug('2');
        leadConvert.setLeadId(LeadIds[0]);

        system.debug('3');
        LeadStatus LeadStat= [SELECT Id, MasterLabel FROM LeadStatus WHERE IsConverted=true LIMIT 1];

        system.debug('4');
        leadConvert.setConvertedStatus(LeadStat.MasterLabel);
        //Leadconvert.setDoNotCreateOpportunity(TRUE); //Remove this line if you want to   create an opportunity from Lead Conversion 

        system.debug('5');
        Database.LeadConvertResult lcr = Database.ConvertLead(leadConvert);
        system.debug('6');
        System.assert(lcr.isSuccess());
        system.debug('7');
    }
}

@isTest
private class AutoConvertLeadsTest {

    @isTest static void testLeadConversion() {
        Lead Ld = new Lead();
        Ld.FirstName = 'TestFirstName';
        Ld.LastName = 'TestLastName';
        Ld.Company = 'TestCompany';
        Ld.LeadSource = 'Web';
        Ld.Rating = 'Warm';
        Ld.Status = 'Open - Not Contacted';
        Ld.Email = 'test@testdwr.com.zz';
        system.debug('a');
        insert Ld;
        system.debug('b');
        Ld.Rating = 'Hot';
        update Ld;
        system.debug('c');

    }
}