smoketest: T5687: simplify "dns forwarding" test setup

Commit eb76729d6324 ("dns forwarding: T5687: Implement ECS settings for PowerDNS
recursor") added a helper "_set_required_options()" method to reduce duplicate
code when setting up the base interface test.

This refactors the test class to call this code always in setUp() so we have
it written only once.

(cherry picked from commit 112376a4ccb96ceee647a4cba5c4f131597b0ea4)