[BQ] Fix insert_overwrite with int + ts partitions

[jtcohen6]

fixes #3063
fixes #3095

Description

In v0.19.0, we added a granularity property of BigQuery partitioned tables. We wrote a general-purpose approach that leveraged the data type + _trunc functions:

https://github.com/fishtown-analytics/dbt/blob/2aa10fb1ed9f012cebb89cd6921b0c124a35aec8/plugins/bigquery/dbt/adapters/bigquery/impl.py#L65

Very cool! Unfortunately, it broke the "dynamic" insert_overwrite incremental strategy with integer- and timestamp-partitioned tables, and we didn't have nearly the automated testing we should have for insert_overwrite given how many community members are using it.

This PR:

• Adds logic handling for int64-type partitions, to avoid writing a (make-believe) int64_trunc function
• Removes the requirement that timestamp and datetime partition values must really be dates (now that BigQuery has removed that requirement, in the form of hour-grain partitioning)
• Performs case-insensitive equality check for partition fields and types (granularities) to avoid hard-refreshing a table if we don't have to. (I can't think of a great way to test for this, as it would require inspecting the logs. Functionally it yields the same result, one is just preferable as an atomic operation.)
• Rework test cases for insert_overwrite + each possible partition type, as well as one case of the static partitions-based approach. For some reason, the old tests (022_bigquery_test/test_scripting.py) just was not running CI! I renamed to test_incremental_strategies.py, which better reflects the set of possibilities.

After merging, I plan to cherry-pick the commit onto 0.19.latest.

Checklist

• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change to the "dbt next" section.

[kwigley]

do you think table.partitioning_type could ever be None here? I'd think table.time_partitioning is not None also means table.partitioning_type has a value, just being nit-picky

[kwigley]

I think this is fine, you can ignore! I dug around docs a little and I think

table.time_partitioning is not None also means table.partitioning_type has a value

holds true

[jtcohen6]

ah good catch though! if one of our intrepid dbt-bigquery beta testers manages to get an error like NoneType has no attribute lower, then we'll know who to blame (me) and where to fix it (here)