Add cadl-ranch/spector tests that validate the emitters gracefully handle optional/missing response headers

We discovered an issue in the C++ emitter, where the generated SDK was not handling response headers which are always supposed to be treated as optional: https://github.com/Azure/autorest.cpp/issues/435

Based on conversation with Laurent, he suggested this is something we can cover in a cadl-ranch test, to make sure all existing (and new) emitters handle missing response headers gracefully.

cc @antkmsft, @lmazuel