Sample code:

@isTest  
private class LeadTriggerTest {  
	  
	static testMethod void testLeadConv() {  
		  
		Lead objLead = new Lead( FirstName = 'Test', LastName = 'Sample', Company = 'Testing Sample Co' );  
		insert objLead;  
		  
		Database.LeadConvert lc = new database.LeadConvert();  
		lc.setLeadId( objLead.Id );  
		lc.setDoNotCreateOpportunity( true );  
		lc.setConvertedStatus( 'Closed – Converted' );  
		  
		Database.LeadConvertResult lcr = Database.convertLead(lc, false);  
		  
		system.debug( 'Errors are ' + lcr.getErrors() );  
		  
		system.assert( lcr.isSuccess() );  
		  
	}  
  
}