@RestResource(urlmapping='/leadconvert/*')
Global class Leadconvert {
     @HttpPost
     global static string post(string firstname,string lastname,string email,string company) {
         List<Lead> lead= [select firstname,lastname,email,company,isconverted from Lead where Firstname=:firstname AND Lastname=:lastname And Email =: email And isconverted=false];
         List<User> use = [select id from User where Username =: email];
         try {
             if(use.size()>0){
                 return 'Given email is already used. please provide another email';
             }
             if(lead.size()>0 && use.size()==0){
                Database.LeadConvert lc = new database.LeadConvert();
                lc.setLeadId(lead[0].id);
                LeadStatus convertStatus = [SELECT Id, MasterLabel FROM LeadStatus WHERE IsConverted=true LIMIT 1];
                lc.setConvertedStatus(convertStatus.MasterLabel);
                Database.LeadConvertResult lcr = Database.convertLead(lc);

                Account a=[select id,name,phone from Account where id=:lcr.getAccountId()];
                Contact cc=[select id,firstname,Lastname,email from Contact where id=:lcr.getContactId()];
                ID ProfileID = [Select id from Profile where id = '00e28000000OBjT'].id;

                User u=new User();
                u.email=cc.email;
                u.contactid = lcr.getContactId();
                u.profileid = profileID;
                u.UserName=email;
                u.Alias=cc.firstname;
                u.CommunityNickName=lastname;
                u.TimeZoneSidKey='America/New_York';
                u.LocaleSidKey='en_US';
                u.EmailEncodingKey='ISO-8859-1';
                u.LanguageLocaleKey='en_US';
                u.FirstName = cc.firstname;
                u.LastName = cc.Lastname;
                insert u;

                return u.id;
             }else {
                 return 'Lead is not exit, please give valid values';
             }
         }catch(exception e){
             return e.getMessage();
         }
    }
}

@isTest
Private class Test_Leadconvert {

    Static TestMethod void  TestPostMethod1() {
        Account acc = new Account(Name = 'Exp');
        insert acc;
        Contact con = new Contact(Lastname = 'Lst Name', AccountId=acc.Id);
        insert con;
        ID ProfileID = [Select id from Profile where id = '00e28000000OBjT'].id;

        User u=new User();
        u.email= 'variation1@gmail.com';
        u.contactid = con.Id;
        u.profileid = profileID;
        u.UserName= 'dummy@gmail.com';
        u.Alias= 'KL';
        u.CommunityNickName= 'perk55';
        u.FirstName = 'Test Name';
        u.LastName = 'KL';
        u.TimeZoneSidKey= 'America/New_York';
        u.LocaleSidKey= 'en_US';
        u.EmailEncodingKey= 'ISO-8859-1';
        u.LanguageLocaleKey= 'en_US';
        insert u;
        System.runAs(u) {
        RestRequest req = new RestRequest();
        RestResponse res = new RestResponse();
        req.requestURI = '/services/apexrest/leadconvert ';
        req.httpMethod = 'POST';
        RestContext.request = req;
        RestContext.response= res;
        String name = Leadconvert.post('Name1','lname','dummy@gmail.com','bizs');
        System.assertEquals('Given email is already used. please provide another email',name);
        }
    }

    Static TestMethod void  TestPostMethod2(){

        Lead leed = new Lead(Firstname='Name2',Lastname='Example2',Email='myorg2@gmail.com',Company='My Org2',Status='Open - Not Contacted');
        insert leed;
        database.leadConvert lc = new database.leadConvert();
        lc.setLeadId(leed.id);
        leadStatus convertStatus = [SELECT Id, MasterLabel FROM LeadStatus WHERE IsConverted=true LIMIT 1];
        lc.setConvertedStatus(convertStatus.MasterLabel);
        Database.LeadConvertResult lcr = Database.convertLead(lc);

        Account a = [select id,name,phone from Account where id=:lcr.getAccountId()];
        Contact con = [select id,firstname,Lastname,email from Contact where id=:lcr.getContactId()];

        User thisUser = [SELECT Id FROM User WHERE Id = :UserInfo.getUserId()];
        ID ProfileID = [Select id from Profile where id = '00e28000000OBjT'].id;
        Id userId;
        System.runAs (thisUser) {
            User u = new User();
            u.email= 'variation1@gmail.com';
            u.contactid = con.Id;
            u.profileid = profileID;
            u.UserName= 'myorg2@gmail.com';
            u.Alias= 'KL';
            u.CommunityNickName= 'perk55';
            u.FirstName = 'Test Name';
            u.LastName = 'KL';
            u.TimeZoneSidKey= 'America/New_York';
            u.LocaleSidKey= 'en_US';
            u.EmailEncodingKey= 'ISO-8859-1';
            u.LanguageLocaleKey= 'en_US';
            insert u;
            userId = u.id;
        }
            RestRequest req = new RestRequest();
            RestResponse res = new RestResponse();
            req.requestURI = '/services/apexrest/leadconvert ';
            req.httpMethod = 'POST';
            RestContext.request = req;
            RestContext.response= res;
            String name = Leadconvert.post('Name2','Example2','dummy2@gmail.com','My Org2');
            System.assertEquals(userId,name); //error is coming here
     }
}