[ADD] test_lint: add test to detect bad usages of _

More often than it should, translations calls are made using
  _("Foo %s" % bar)

instead of
  _("Foo %s") % bar

or the even better (but more recent)
  _("Foo %s", bar)

The issue of using the first writing is that the translation lookup
won't work at the execution (looking for ir.translation for src "Foo
Alice", "Foo Bob",... instead of "Foo %s").

Add a pylint test to ensure this won't happen anymore

odoo/addons/test_lint/tests/_odoo_checker_gettext.py

+import os
+
+import astroid
+from pylint import checkers, interfaces
+
+
+class OdooBaseChecker(checkers.BaseChecker):
+    __implements__ = interfaces.IAstroidChecker
+    name = 'odoo'
+
+    msgs = {
+        'E8502': (
+            'Bad usage of _, _lt function.',
+            'gettext-variable',
+            'See https://www.odoo.com/documentation/14.0/reference/translations.html#variables'
+        )
+    }
+
+    @checkers.utils.check_messages('gettext-variable')
+    def visit_call(self, node):
+        if isinstance(node.func, astroid.Name) and node.func.name in ('_', '_lt'):
+            first_arg = node.args[0]
+            if not (isinstance(first_arg, astroid.Const) and isinstance(first_arg.value, str)):
+                self.add_message('gettext-variable', node=node)
+
+
+def register(linter):
+    linter.register_checker(OdooBaseChecker(linter))

odoo/addons/test_lint/tests/test_pylint.py

@@ -31,6 +31,7 @@ class TestPyLint(TransactionCase):
 
         # custom checkers
         'sql-injection',
+        'gettext-variable',
     ]
 
     BAD_FUNCTIONS = [
@@ -68,7 +69,7 @@ class TestPyLint(TransactionCase):
             '--enable=%s' % ','.join(self.ENABLED_CODES),
             '--reports=n',
             "--msg-template='{msg} ({msg_id}) at {path}:{line}'",
-            '--load-plugins=pylint.extensions.bad_builtin,_odoo_checker_sql_injection',
+            '--load-plugins=pylint.extensions.bad_builtin,_odoo_checker_sql_injection,_odoo_checker_gettext',
             '--bad-functions=%s' % ','.join(self.BAD_FUNCTIONS),
             '--deprecated-modules=%s' % ','.join(self.BAD_MODULES)
         ]