Rename driver/test namespaces in the PHP files

[aik099]

The namespaces of the PHP classes (both src and tests folders) need to be standardized to match other Mink drivers:

• namespace for the src folder: Behat\Mink\Driver;
• namespace for the tests folder: Behat\Mink\Tests\Driver.

[stof]

No. Using Mink as the top-level namespace was an explicit choice from me.

I won't go in the process of renaming the namespace from Behat\Mink to Mink in all existing package (the BC break for the ecosystem is not worth it), but I'd like to use Mink as top-level namespace for any new package going forward (as Mink is no longer under the Behat umbrella since a while, when we separated the organizations). That's also why we use mink/* and not behat/mink-* for the composer package name.

[aik099]

Good to know. Thank you.