I have modified your class and test class a bit. Its bettter to have seperate test class: 
Class: 

public class chatterTooltip {
    public string uID   { get; set; }
    
    public User u       { get; set; }
    
    public integer followers    { get; set; }
    public integer following    { get; set; }
    
    public boolean followingme  { get; set; }
    
    public string lastUpdate    { get; set; }
    public string timestr       { get; set; }
    
    public boolean isTest       { get; set; }
    
    public chatterTooltip() {
        //isTest = false;
    }
    
    public string getUserName() {
        if( uID == null )
            return null;
            
        if( u != null )
            return u.name;
            
        User[] us = [select id, name from User where id = :uID];
            
        if( us.isEmpty() )
            return '';
            
        u = us[0];
            
        loadData();
            
        return u.name;
    }
    
    public void loadData() {
        integer qlimit = ( isTest ? 1 : 1000 );
        
        EntitySubscription[] ess = [select id, parentid from EntitySubscription where parentid = :uID or subscriberid = :uID limit :qlimit];
            
        followers = 0;
        
        for( EntitySubscription es : ess )
            if( es.parentid == uID )
                followers++;
                
        following = ess.size() - followers;
        
        if( UserInfo.getUserId() != uID ) {
            EntitySubscription[] ess2 = [select id from EntitySubscription where parentid = :UserInfo.getUserId() and subscriberid = :uID];
            followingme = ess2.size() > 0;
        } else
            followingme = false;
            
        UserFeed[] ufs = [select id,  createddate from UserFeed where parentid = :uID order by createddate desc limit 1];
            
        if( ufs.isEmpty() )
            return;
            
        //lastUpdate = ufs[0].FeedPost.body;
        
        if( lastUpdate == null )
            return;
        
        if( lastUpdate.length() > 70 )
            lastUpdate = lastUpdate.substring( 0, 70 ) + '...';
            
        timestr = relativeTime( ufs[0].createddate );
    }
    
    public string relativeTime( Datetime dt ) {
        long diff =  ( Datetime.now().getTime() - dt.getTime() ) / 1000;
        string unit;
        
        if( diff < 60 )
            unit = 'second';
        else if( diff < 60 * 60 ) {
            diff /= 60;
            unit = 'minute';
        } else if( diff < 60 * 60 * 24 ) {
            diff = diff / 60 / 60;
            unit = 'hour';
        } else {
            diff = diff / 60 / 60 / 24;
            unit = 'day';
        }
        
        if( diff > 1 )
            unit += 's';
            
        return diff + ' ' + unit + ' ago';
    }
    

}

Test Class:

@isTest
private class ChatterToolTipTest {
    @isTest
    private static void runTest(){
    
            //Find user with Profile = Sales and Service
        Profile sysadm = [Select id from Profile where Name = 'System Administrator' limit 1];
        User u = new User(
            Alias = 'standt', 
            Email='standarduser@testorg.com',
            EmailEncodingKey='UTF-8',
            LastName='Testing',
            LanguageLocaleKey='en_US',
            LocaleSidKey='en_US',
            ProfileId = sysadm.Id,
            TimeZoneSidKey='America/Los_Angeles',
            UserName='Test123_tesst@testorg.com'
            );
        
        
        Test.startTest();
        insert u;
        u.aboutme = 'Test about me';
        update u;
        chatterTooltip ct = new chatterTooltip();
        ct.isTest = true;
        ct.uid = u.id;
        ct.lastUpdate ='TestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTest';
        ct.timestr ='Test';
        
        ct.getUserName();
        ct.relativeTime(datetime.now());      
        ct.loadData(); 
        
        ct.uid = userInfo.getUserId();
        ct.loadData();
                    
        Test.stopTest();
      }
}

​This should get you 80% code. I had to modify a bit code as FeedPost.Body is not supported in new Orgs.