Fix phpunit.xml.dist in new directory structure

- [X] The bootstrap setting of phpunit wasn't updated in the new directory structure.
- [X] We need to add the `<php><server name="KERNEL_DIR" value="app/" /></php>` part because the [KernelTest::getKernelClass()](https://github.com/symfony/symfony/blob/master/src/Symfony/Bundle/FrameworkBundle/Test/KernelTestCase.php#L101) search by default the Kernel in the same directory than phpunit.xml.dist, which is not the case anymore

Any opinion on this @fabpot or @romainneutron?