[FIX] core: correctly read test_tags in config file

The test_tags is stored as None by default in the config file,
but 'None' is interpreted as a string when read.

This is a dirty fix in order to avoid to launch tests when using
a config file, we should do something cleaner in master and refactor
part of this code (substitute 'None' in the load method as it is
done for boolean)

closes #27498

closes odoo/odoo#27564

odoo/tools/config.py

@@ -394,6 +394,9 @@ class configmanager(object):
         # the same for the pidfile
         if self.options['pidfile'] in ('None', 'False'):
             self.options['pidfile'] = False
+        # the same for the test_tags
+        if self.options['test_tags'] == 'None':
+            self.options['test_tags'] = None
         # and the server_wide_modules
         if self.options['server_wide_modules'] in ('', 'None', 'False'):
             self.options['server_wide_modules'] = 'base,web'