[IMP] service: Display a warning when launching tests in multi-workers mode

Indeed, running the testsuite is not supported in multi-process (worker/pre-fork) mode, for multiple reasons: no multiple/concurrent execution of tests, necessity for in-memory cross-request statefulness (test cursor), etc.

We have no plan to work around these issues, as there seems to be no strong use case.
Our testsuite is mainly focused on the business/framework aspects, and not scalability/deployment aspects - which would require a much more complicated test infrastructure.

odoo/service/server.py

@@ -952,6 +952,9 @@ def start(preload=None, stop=False):
     if odoo.evented:
         server = GeventServer(odoo.service.wsgi_server.application)
     elif config['workers']:
+        if config['test_enable'] or config['test_file']:
+            _logger.warning("Unit testing in workers mode could fail; use --workers 0.")
+
         server = PreforkServer(odoo.service.wsgi_server.application)
     else:
         server = ThreadedServer(odoo.service.wsgi_server.application)