For now, tests have to do the following in order to start a remote server:
https://searchfox.org/mozilla-central/rev/7c20ad925005fbad7b8e08813115f1ec7fa1c248/remote/test/browser/browser_cdp.js#30-33

  // Start the CDP server
  const RemoteAgent = Cc["@mozilla.org/remote/agent"].getService(Ci.nsISupports).wrappedJSObject;
  RemoteAgent.tabs.start();
  RemoteAgent.listen(Services.io.newURI("http://localhost:9222"));

The first line, to retrieve the RemoteAgent singleton is going to be simplified in bug 1536888. But there will still be the second line, where the tests have to know that internally, the remote agent has to start its TabsObserver.
It would be helpful if that step can be done as part of listen or something.

https://treeherder.mozilla.org/#/jobs?repo=try&revision=276e7d66564c7045eb8ed36224a26a783811765c

Start the TabObserver from RemoteAgent.listen in order to prevent
having the tests to manually start it.
Start it from listen instead of init as we do stop the TabObserver from
stop and stop is the reverse of listen, not the reverse of init.

Automatically initialize the agent from listen in order to only
have to call this one method in order to start it.

This allows improving all tests exceptions without requiring
any specific to be done in each tests.
I'm also moving a few imports to head.js to share the most into head.js.

https://hg.mozilla.org/mozilla-central/rev/216940ec115d
https://hg.mozilla.org/mozilla-central/rev/6586c6e89759