[IMP] tests: verify that the browser is running

When issuing the first json command, it happens that the chrome process
has crashed. As a consequence, a log entry appears with saying that it
could not connect to chrome debugger.

After some investigations, it appeared that it was only happening with
google-chrome versions 75.0.3770.90 and 74.0.3729.169.

This error message was seen on the chrome stderr:
listp->slotinfo[cnt].gen <= GL(dl_tls_generation).

As a conclusion, it seems that we were hit by this issue:
https://github.com/GoogleChrome/puppeteer/issues/2207

While this commit does not fix the google-chrome issue, it adds a new
verification to check if google-chrome process is running before trying
to issue the json command. If it's not case, an error message is logged
with the error code.

odoo/tests/common.py

@@ -638,6 +638,9 @@ class ChromeBrowser():
         tries = 0
         failure_info = None
         while tries * delay < timeout:
+            if self.chrome_process.poll() is not None:
+                self._logger.error('Chrome crashed at startup with return code', self.chrome_process.returncode)
+                break
             try:
                 r = requests.get(url, timeout=3)
                 if r.ok: