test: Refactor utility functions verify_data and verify_schema

By wrapping them into a container class `AssertionHelper`, it is easy
to parameterize them, and to provide them to the test functions using
a pytest fixture.

This way, they are reusable from database adapter implementations which
derive from PostgreSQL.

The motivation for this is because the metadata column prefix `_sdc`
needs to be adjusted for other database systems, as they reject such
columns, being reserved for system purposes. In the specific case of
CrateDB, it is enough to rename it like `__sdc`. Sad but true.

Author: amotl