Reduce running time of jsonb_sqljson test

Reduce running time of jsonb_sqljson test

The test created a 1m row table in order to test parallel operation of
JSON_VALUE. However, this was more than were needed for the test, so
save time by halving it, and also by making the table unlogged.
Experimentation shows that this size is only a little above the number
required to generate the expected output.

Per gripe from Andres Freund

Discussion: https://postgr.es/m/20220406022118.3ocqvhxr6kciw5am@alap3.anarazel.de