Add Support for Sandbox execution

[DavidLambauer]

Turns out that the cli_arg_auto_proxy_di.xml is not added to the sandbox directories. Whenever you run integration tests via php bin/magento dev:tests:run integration or via cd dev/tests/integration && php ../../../vendor/bin/phpunit -c ../../../phpunit-integration.xml Magento create temporary directories under dev/tests/integration/tmp/sandox-..... These sandbox directories don't include the cli_arg_auto_proxy_di.xml, so the classic flag error occurs.

In \Magento\TestFramework\Application::copyAppConfigFiles, the pattern shows that we could place the cli_arg_auto_proxy_di.xml into app/etc/SOMEDIR/di.xml and it would work for the sandbox tests.

This needs to be tested for the non-sandbox execution though.

[vpodorozh]

resolved with #15