Move data test "rendering" out of python compilation step

[kwigley]

Description

We currently wrap data tests with a CTE during the node compilation step
https:/fishtown-analytics/dbt/blob/24e4b75c354e0f6bc5af7b51c4bb220428c84d5b/core/dbt/compilation.py#L362-L372
This ties our hands for desired changes to tests. We should be able to align data tests with how models are compiled using a test materialization. The goal of this issue is to maintain current implementation and reporting of data tests, but compile them similarly to models.

This will affect execution as well, since we execute the sql directly and simply return the amount of rows in the result.
https:/fishtown-analytics/dbt/blob/24e4b75c354e0f6bc5af7b51c4bb220428c84d5b/core/dbt/task/test.py#L82-L83
We can use prior art of model execution to help here as well.