test for Concatenable in ORM evaluator for concat_op

test for Concatenable in ORM evaluator for concat_op

Fixed issue in ORM evaluator where two datatypes being evaluated with the
SQL concatenator operator would not be checked for
:class:`.UnevaluatableError` based on their datatype; this missed the case
of :class:`_postgresql.JSONB` values being used in a concatenate operation
which is supported by PostgreSQL as well as how SQLAlchemy renders the SQL
for this operation, but does not work at the Python level. By implementing
:class:`.UnevaluatableError` for this combination, ORM update statements
will now fall back to "expire" when a concatenated JSON value used in a SET
clause is to be synchronized to a Python object.

Fixes: #11849
Change-Id: Iccd97edf57b99b9a606ab3a47d2e3e5b63f0db07