Add integration tests

[jtcohen6]

We don't currently have any integration tests. Until we do, we need to rely on manual end-to-end testing and the (currently small) community of Presto users to validate proposed changes.

There's much more to think about here:

• How can we set up a reasonable testing environment for Presto? Running everything via the memory catalog is a fine way to check syntax, but it's probably not a good representation of how people run Presto in the wild.
• The dbt-spark plugin uses a modified version of https://github.com/fishtown-analytics/dbt-integration-tests. The dbt-presto plugin does not support incremental models. How might dbt-integration-tests (or dbt-core) define a matrix of available features and associated tests, such that each adapter can declare which ones it does (or doesn't) support?

[jtcohen6]

There are two dbt-core considerations relevant to this issue:

• dbt-core itself includes an integration test dedicated to Presto
• Refactoring the core test suite to support plugins inheriting/disabling/overriding specific tests (Refactor test suite to support cross-plugin testing dbt-core#2204)

[jtcohen6]

Closing this, now that we have some integration tests running in CI. The better answer requires significant work in dbt-core, which should stem from the issue linked above.