Force the product to have pses on free product test

Sometimes on the tests, we have the error
```
............................................................... 63 / 783 ( 8%)
............................................................... 126 / 783 ( 16%)
............................................................... 189 / 783 ( 24%)
..........................FSS.................................. 252 / 783 ( 32%)
............................................................... 315 / 783 ( 40%)
............................................................... 378 / 783 ( 48%)
............................................................... 441 / 783 ( 56%)
...................................F..........................PHP Fatal error: Call to a member function getPricesByCurrency() on a non-object in /home/travis/build/thelia/thelia/tests/phpunit/Thelia/Tests/Coupon/Type/FreeProductTest.php on line 48
```

The problem may come from the DB. The chosen product may not have any default pse. So the ```getDefaultSaleElements()``` method returns null.