fix: Add -wait-for-browser

As of about Firefox 64, on Windows Firefox uses a separate launcher
process which then spawns the browser process. The launcher process will
terminate quickly causing karma to think something failed.

As with Mozilla's own automation tests we should use -wait-for-browser
to make the launcher process wait for the browser process to complete
and return its exit code.

Unfortunately, killing the launcher process won't kill the browser
process so this isn't a complete solution. However, killing the browser
process is likely more complicated while simply getting the browser to
launch at all is probably enough for local testing.

index.js

@@ -147,7 +147,7 @@ var FirefoxBrowser = function (id, baseBrowserDecorator, args) {
     fs.writeFileSync(profilePath + '/prefs.js', this._getPrefs(args.prefs))
     self._execCommand(
       command,
-      [url, '-profile', profilePath, '-no-remote'].concat(flags)
+      [url, '-profile', profilePath, '-no-remote', '-wait-for-browser'].concat(flags)
     )
   }
 }