Chore: Reorganize engine adapter integration tests

[erindru]

Currently, in order to write an integration test against one of the engines, it needs to be tacked on to test_integration.py.

This file is starting to get a bit unwieldy:

• It's ~2700 lines and growing
• Every test using TestContext automatically gets 3 versions: df, query and pyspark. Often you only want one of these, typically query (and almost never pyspark) leading to a bunch of skipping logic within tests if the test_type is not query
• Every new test has to go into this file, even if its only for a specific database, which means you have to add skipping logic to ensure its being invoked for the correct database
• It sits at the same level in the hierarchy as the unit tests for each engine adapter, making it slightly confusing as to where new tests should go

This PR makes the following changes:

• Moves everything related to the engine adapter integration tests into an integration subfolder
• Moves the engine adapter / TestContext setup logic into a common place
• Demonstrates adding an integration test file scoped to a specific engine (to be extended on in future PR's)

The goal here is to set the foundation for breaking these up further in future. Tests affecting all adapters can stay in test_integration.py and then tests for a specific database can be put in test_integration_<database>.py.

The immediate motivation was needing somewhere to put tests for the upcoming partition management logic in the Athena and Postgres adapters but these changes also give the option of moving eg the pyspark tests out of the main file and into a Spark-specific area

[eakmanrq]

Was this intentional?

[erindru]

Yes, I needed to show that all the tests were still running / passing (if there was an issue with these, it would only show up post-merge)

I'll uncomment this prior to merge

[eakmanrq]

Thanks for laying the foundation for improving this!