[3.3][Tests] Silex2 deprecations

[GwendolenLynch]

In Pimple v3, once the container element has been accessed, it is "frozen" to change

e.g.

$app = new \Pimple\Container();

$app['koala'] = 'drop bear';
$value = $app['koala'];

// This will now throw an exception
$app['koala'] = 'kenny';

This is Phase I of getting tests in order 😉

[bobdenotter]

Cool stuff! 👍