The Netmonitor used to call the WebConsoleFront `getString` method to get the full
text of a long string. This could make some test to fail with Fission and target
switching.
Since the webconsole actor is not even involved in the webconsole method, there's
no reason to keep using it. A similar function is added into a util file; the only
difference is that it doesn't cache the front it creates, which does not matter
as the netmonitor only calls this once and then put the full text in its redux
store.
This allows us to re-enable `browser_ext_devtools_network.js` with Fission.
Differential Revision: https://phabricator.services.mozilla.com/D120333
Alexandre Poirot