[FIX] tests: avoid confusing traceback at browser start

During an HttpCase, when the Chrome browser starts, it may happens that
the websocket is unreachable. In such a case, the stop method is called
and tries to stop the browser gracefully by sending a `Browser.close`
through the websocket ... which is not reachable.

The result is a confusing traceback. With this commit, nothing is sent
is the websocket is not available.

Moreover, the `sigxcpu_handler` attribute is used in the stop method but
not yet declared. Fixed by moving it before calling the _chrome_start
method.

X-original-commit: 0d156fd3bc114b40680ca5aa0456e0f21a5b8cff

odoo/tests/common.py

@@ -644,6 +644,7 @@ class ChromeBrowser():
         os.makedirs(self.screenshots_dir, exist_ok=True)
 
         self.window_size = window_size
+        self.sigxcpu_handler = None
         self._chrome_start()
         self._find_websocket()
         self._logger.info('Websocket url found: %s', self.ws_url)
@@ -652,7 +653,6 @@ class ChromeBrowser():
         self._websocket_send('Runtime.enable')
         self._logger.info('Chrome headless enable page notifications')
         self._websocket_send('Page.enable')
-        self.sigxcpu_handler = None
         if os.name == 'posix':
             self.sigxcpu_handler = signal.getsignal(signal.SIGXCPU)
             signal.signal(signal.SIGXCPU, self.signal_handler)
@@ -811,6 +811,8 @@ class ChromeBrowser():
         """
         send chrome devtools protocol commands through websocket
         """
+        if self.ws is None:
+            return
         sent_id = self.request_id
         payload = {
             'method': method,