tests: Robustify testNICAdd by creating the test Virtual Network before page load
tests: Robustify testNICAdd by creating the test Virtual Network before page load

It's also possible to create the test network after the page was loaded
(how we used to do it) and just wait for the the internal state to get updated,
before opening the dialog which uses the newly added resource, but this is
prone to be more unstable than just creating the necessary resources
before page load.

Fixes #13861
Closes #13899
deryni/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,deryni/cockpit,cockpituous/cockpit,mvollmer/cockpit,cockpituous/cockpit,martinpitt/cockpit,deryni/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,garrett/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,garrett/cockpit,cockpituous/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpituous/cockpit,garrett/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,garrett/cockpit,deryni/cockpit,garrett/cockpit,martinpitt/cockpit,cockpituous/cockpit,mvollmer/cockpit,deryni/cockpit,deryni/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,deryni/cockpit