[CT-1630] Convert column_types test for dbt-bigquery

[McKnight-42]

resolves #dbt-labs/dbt-core#6404

Description

convesion of 056 to new functional test method

todo before merge: revert dev_requirements dbt-core pin

Checklist

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[mikealfare]

Is there a reason to separate this from test_alter_column_types.py? The two fixtures (module level variables) are copies of fixtures in the other file. So the only new content is the class with two methods.

I would keep the name test_column_types.py as it's more generic, so I guess I'm saying to move the other file's contents here technically.

[McKnight-42]

These are separated for now as it is only currently tested in bigquery and is how it was historically, this does also raise a fine question of how big of a change in functionality a test is doing should they be before they are their own file/test but still grouped together.

[VersusFacit]

I'd suggest just revising this to have the fixtures in the same file, then import them, thereby reducing the duplicate code. Best of both worlds.

Nit: do that

[McKnight-42]

There are a few very slight differences in the fixtures for each test, one easily fixed by a project_config_update in that we materialize alt model as a table, the second for the schema being a change in the int64 type