Improve a bit the tests of pg_walinspect

Improve a bit the tests of pg_walinspect

This commit improves the tests of pg_walinspect on a few things:
- Remove aggregates for queries that should fail.  If the code is
reworked in such a way that the behavior of these queries is changed,
we would get more input from them, written this way.
- Expect at least one record reported in the valid queries doing scans
across ranges, rather than zero records.
- Adjust a few comments, for consistency.

Author: Bharath Rupireddy
Discussion: https://postgr.es/m/CALj2ACVaoXW3nJD9zq8E66BEf-phgJfFcKRVJq9GXkuX0b3ULQ@mail.gmail.com