Always expire wfdMeta resources in test per default

[relthyg]

We set

    webfactory_wfd_meta.always_expire_wfd_meta_resources: true

in our Symfony-default-config in https://github.com/webfactory/webfactory-symfony-default-config/pull/14, but we can't actually do that, because this triggers an error in projects not using webfactory/wfdmeta-bundle (see https://github.com/webfactory/webfactory-symfony-default-config/pull/17)

This PR sets the configuration parameter webfactory_wfd_meta.always_expire_wfd_meta_resources to true as default in test environments end keeps it false everywhere else.

Closes #35

[mpdude]

Sorry to say, but I don't like this approach. This is not how the Symfony configuration mechanism works or what typical Configuration classes look like, and it hard-codes the assumption about the name of an environment variable and the environment name into that class.

The env variable name or environment name may be the Symfony default or best practice, but that is just a convention.