[CT-808] Add "adapter zone" tests for grants

[leahwicz]

• test that projects can define grants in 1) the dbt_project.yml, 2) models, seeds, and snapshots, and 3) both the dbt_project.yml and models, seeds, and snapshots and those permissions can be read back directly from the postgres warehouse after a run.
• test two subsequent runs (dbt build && dbt build) with: same grants; additional grants in second run; fewer grants in second run. Confirm that the final set of grants matches the final configured set of grants in all cases.
  • Special case: grants: {'select': []} in second run. Should revoke all grants, and add none back.
• For models: tests for view, table, and incremental materializations
• For incremental: subsequent runs with and without --full-refresh
• Read back logs and count number of show/grant/revoke statements, to validate that dbt is taking the most efficient possible route — this is half done, would require more logic to handle per-adapter behavior
• Test cases for known failure conditions:
  • granting to a user/role/group that doesn't exist
  • granting a privilege type that doesn't exist
• Multiple grantees
• Multiple privileges

Adapter specific

These tests will need to be defined in each individual adapter plugin, as new tests or extensions of existing "adapter zone" ones

Snowflake

• Test interaction with copy_grants for view, table, incremental (--full-refresh), seed (--full-refresh)

BigQuery

• Test anticipated failure when location is not defined — see Alt approach for grant location dbt-bigquery#218

Spark

• Test anticipated failure when running against OSS Apache Spark (does not support grant SQL statement)