Do not wait for icons to load before interacting with a combo box (#9717)

Do not wait for icons to load before interacting with a combo box (#9717)

Icon load detection code does not handle error events so broken images
will cause tests to fail forever. In other cases also, it seems very
rare to need icons to be loaded before interacting with a combo box. If
there really are use cases for this, it would be better to have a method
to query if all icons are loaded so that special tests can use this method.

Fixes https://github.com/vaadin/testbench/issues/916