common: Fix test-webserver for environments with only loopback interface
common: Fix test-webserver for environments with only loopback interface

If there is no non-loopback interface (which is common in e. g. mock
nspawn environments), leave `TestCase.hostport` as `NULL` instead of
crashing with an assertion. Skip the tests (or parts of them) which
require an external address.

Fixes #11312
Closes #11314

deryni/cockpit,mvollmer/cockpit,cockpit-project/cockpit,deryni/cockpit,cockpituous/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,deryni/cockpit,martinpitt/cockpit,cockpituous/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,mvollmer/cockpit,deryni/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,deryni/cockpit,martinpitt/cockpit,garrett/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,mvollmer/cockpit,deryni/cockpit,garrett/cockpit,cockpit-project/cockpit,garrett/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,garrett/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,martinpitt/cockpit,deryni/cockpit,cockpituous/cockpit,garrett/cockpit,martinpitt/cockpit