[IMP] *: add generic make_jsonrpc method in tests

*: bus, crm_livechat, crm_mail_plugin, im_livechat, payment, payment_demo,
pos_online_payment, test_discuss_full, test_mail_full, tests, website_livechat,
website_sale.

Several python tests have their own way to make jsonrpc requests. This PR adds a
generic `make_jsonrpc_request` method to the `HttpCase` in order to provide a
generic way to do so.

At the same time, calls to `_open_livechat_channel` are removed in favor of
jsonrpc request to `get_session`. This makes the tests more realistics (some
incoherences were present like passing `country_id` to the open channel method
while the user country id is not set...)

Finally, this will ease the diff in the PR introducing livechat visitors as mail
guests.

part of task-3332628

closes odoo/odoo#130036

Related: odoo/enterprise#44840
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>

odoo/tests/common.py

@@ -1610,6 +1610,12 @@ class Transport(xmlrpclib.Transport):
         return super().request(*args, **kwargs)
 
 
+class JsonRpcException(Exception):
+    def __init__(self, code, message):
+        super().__init__(message)
+        self.code = code
+
+
 class HttpCase(TransactionCase):
     """ Transactional HTTP TestCase with url_open and Chrome headless helpers. """
     registry_test_mode = True
@@ -1812,6 +1818,34 @@ class HttpCase(TransactionCase):
             return sup.profile(description=request.httprequest.full_path)
         return profiler.Nested(_profiler, patch('odoo.http.Request._get_profiler_context_manager', route_profiler))
 
+    def make_jsonrpc_request(self, route, params=None, headers=None):
+        """Make a JSON-RPC request to the server.
+
+        :param str route: the route to request
+        :param dict params: the parameters to send
+        :raises requests.HTTPError: if one occurred
+        :raises JsonRpcException: if the response contains an error
+        :return: The 'result' key from the response if any.
+        """
+        data = json.dumps({
+            'id': 0,
+            'jsonrpc': '2.0',
+            'method': 'call',
+            'params': params,
+        }).encode()
+        headers = headers or {}
+        headers['Content-Type'] = 'application/json'
+        response = self.url_open(route, data, headers=headers)
+        response.raise_for_status()
+        decoded_response = response.json()
+        if 'result' in decoded_response:
+            return decoded_response['result']
+        if 'error' in decoded_response:
+            raise JsonRpcException(
+                code=decoded_response['error']['code'],
+                message=decoded_response['error']['data']['name']
+            )
+
 
 def no_retry(arg):
     """Disable auto retry on decorated test method or test class"""