[Feature] Implement adapter tests in dbt-postgres

[colin-rogers-dbt]

Is this your first time submitting a feature request?

• [X] I have read the expectations for open source contributors
• [X] I have searched the existing issues, and I could not find an existing issue for this feature
• [X] I am requesting a straightforward extension of existing dbt-postgres functionality, rather than a Big Idea better suited to a discussion

Describe the feature

Currently dbt-postgres has implemented dbt-core's tests but doesn't reference the dbt-tests-adapter tests.

Describe alternatives you've considered

No response

Who will this benefit?

No response

Are you interested in contributing this feature?

No response

Anything else?

No response

### Tasks
- [ ] https://github.com/dbt-labs/dbt-adapters/pull/93
- [ ] https://github.com/dbt-labs/dbt-postgres/pull/18

[mikealfare]

Re-opening to resolve issues that were discovered in the original migration.