Laravel 5.2 - phpunit - Cannot modify header information - headers already sent by

Can you please help me solve this issue on my local homestead/vagrant machine?

When I run the command: PHPUnit

I getting this result

PHPUnit 4.8.36 by Sebastian Bergmann and contributors.

E
Time: 1.97 seconds, Memory: 6.00MB

                                                                                                               There was 1 error:                                                    

                                                                                                               1) ExampleTest::testBasicExample                                      

Cannot modify header information - headers already sent by (output started at /home/vagrant/Code/Project/vendor/php unit/phpunit/src/Util/Printer.php:134)

                                                                                                               /home/vagrant/Code/Project/bootstrap/app.php:4                         

/home/vagrant/Code/Poptin/tests/TestCase.php:20
/home/vagrant/Code/Project/vendor/laravel/framework/src/Illuminate/Foundation/Testing/TestCase.php:85 /home/vagrant/Code/Project/vendor/laravel/framework/src/Illuminate/Foundation/Testing/TestCase.php:64

Please help! I've tried everything!!!

via Chebli Mohamed