Add tox

[emmyoop]

resolves #920
resolves #948

Problem

We're using bash to run tests right now and want to standardize on tox so all tests can be run the same, regardless which method is being used for testing.

Solution

2. Move /integration_tests/ci/sample.profiles.yml to /integration_tests and rename it to profiles.yml
3. Rename env vars to be more standardized + update *.env files accordingly
4. Add env var for schemas
5. Update *.env files accordingly
6. Add supported_adapters var and [testenv:list_supported_adapters] tox command to be able to determine what env vars to test. This allows the package maintainer to explicitly control what adapters should be tested without inferring it from the profile. This is useful during development or for iterative changes.
7. Add tox.ini for running current tests.
8. Update CircleCI test script to stop copying the profile over
9. Added a github workflow to run tests in GitHub. CircleCI was left in place so we can verify tests for a small overlap period.

I did not rewrite the tests to use tox as it was out of scope for this exercise.

Checklist

• This code is associated with an issue which has been triaged and accepted for development.
• I have read the contributing guide and understand what's expected of me
• 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 README.md (if applicable)

[dbeatty10]

Looks good to me.

We'll do a follow-up later to remove the CircleCI workflows and just use CI via GitHub Actions.

[emmyoop]

Related to dbt-labs/dbt-core#10668