public List<Case> excase{get;set;}
 public String radioValue{get;set;}
 public Pagereference updateCase() {
  radioValue = apexpages.currentpage().getparameters().get('group');
  showtable=true;
  case cd = [select Id from Case where Id = : radioValue];
  cd.Last_Verified_by_User__c = UserInfo.getUserId();
  cd.Last_Verified_Date_Time__c = datetime.now();
  update cd;
  Pagereference ref = new PageReference('/' + cd.Id);
  return ref;
 }

<apex:pageblockTable value="{!excase}" var="exscase" id="caselist" rendered="{!showtable}">
            <apex:column headervalue="Select Case">
                <input type="radio" name="group" value="{!exscase.Id}" />
            </apex:column>
               <apex:column value="{!exscase.CaseNumber}"/>
               <apex:column value="{!exscase.Status}"/>
               <apex:column value="{!exscase.Subject}"/>

            </apex:pageblockTable>

@isTest
static void testupdatecase(){
   Contact ct= new Contact (Firstname='test_firstname',LastName = 'test_lastname');
   CreateNewCase caseobject=new CreateNewCase(new ApexPages.StandardController (ct));
   insert ct;
   caseobject.showtable=true;
   caseobject.createCase();
 //  struck here - caseobject.radioValue=[select id from case where contactid=:ct.id];
   PageReference resPg = caseobject.updateCase();



}