Before you contribute make sure you install all necessary dependencies using:
$ composer install
To run tests simply run the phpunit
executable in the vendor/bin
$ ./vendor/bin/phpunit
You should get an output similar to this
$ ./vendor/bin/phpunit
PHPUnit 4.8.29 by Sebastian Bergmann and contributors.
..........................SSSSSS............................... 63 / 157 ( 40%)
............................................................... 126 / 157 ( 80%)
...............................
Time: 658 ms, Memory: 25.00MB
OK, but incomplete, skipped, or risky tests!
Tests: 157, Assertions: 207, Skipped: 6.