added failing German mobile number to tests

[mgrundkoetter]

Formatting this mobile number will fail right now as the leading zero is missing in the national format. This is just an example that should proof the issue #12 can not be fixed by the proposal there in case it is a certain mobile carrier prefix and not a fixed landline.

The error in formatting comes from the underlying library (https://libphonenumber.appspot.com/phonenumberparser?number=%2B491520123123&country=DE&geocodingLocale=de-DE). I filed an issue there but as soon as it get fixed there, adding a test here makes sense to me.

https://issuetracker.google.com/issues/247105388