2 answers

You can manipulate the test class like this:- @isTest
public class TestOppExtension {
public static testmethod void testOppExtensionMethod(){
Opportunity opp = new Opportunity(Name='Test Opp', StageName = 'Prospecting', CloseDate = System.today());
insert opp;
ApexPages.StandardController stdController = new ApexPages.StandardController(opp);
OppExtension ocontroller = new OppExtension(stdController );
ocontroller.s1 = 'North America (%) : ';
ocontroller.s2 = 'APAC (%) : ';
ocontroller.s3 = 'LATAM (%) : ';
PageReference pr = ocontroller.assignvalue();
System.assertEquals('', ocontroller.s1);
}
}