Refactor dialect tests for combinations

Refactor dialect tests for combinations

Dialect tests tend to have a lot of lists of types,
SQL constructs etc, convert as many of these to @combinations
as possible.

This is exposing that we don't have per-combination
exclusion rules set up which is making things a little bit
cumbersome.

Also set up a fixture that does metadata + DDL.

Change-Id: Ief820e48c9202982b0b1e181b87862490cd7b0c3