Doc: run tests with code coverage

[aaa2000]

Without code coverage,

Failure (17 tests, 32/32 methods, 0 void method, 0 skipped method, 0 uncompleted method, 0 failure, 32 errors, 0 exception)!
> There are 32 errors:

=> M6Web\Bundle\LogBridgeBundle\Tests\Units\Config\Configuration::testConfiguration():
==> Error E_WARNING in xxxx/vendor/atoum/atoum/classes/test.php on line 1344, generated by file xxxxe/vendor/atoum/atoum/classes/test.php on line 1344:
Code coverage needs to be enabled in php.ini by setting 'xdebug.mode' to 'coverage'

=> M6Web\Bundle\LogBridgeBundle\Tests\Units\Config\Filter::testFilter():
==> Error E_WARNING in xxxx/vendor/atoum/atoum/classes/test.php on line 1344, generated by file xxx/vendor/atoum/atoum/classes/test.php on line 1344:
Code coverage needs to be enabled in php.ini by setting 'xdebug.mode' to 'coverage'

....