Next parallelism improvements will allow pgloader to use more than one
COPY thread to load data, with the impact of changing the order of rows
in the database.
Rather than doing a copy out and `diff` of the data just loaded, load
the reference data and do the diff in SQL:
select * from loaded.data
except
select * from expected.data
If such a query returns any row, we know we didn't load what was
expected and the regression test is failing.
This regression testing facility should also allow us to finally add
support for multiple-table regression tests (sqlite, mysql, etc).
Those tests currently only work when a single table is the target of the
load, and when this target is explicit in the INTO target clause. More
work needs to be done to cover interesting cases like MySQL and SQLite
where we want to diff a full database rather than a single table.
