Add tox support for testing redshift, bigquery and snowflake

[emmyoop]

Resolves #316

Description & motivation

This PR make a slight update to the current testing layout to allow dbt Labs to start testing this package as part of our own
release process.

This only adds support for testing with snowflake and redshift.

Adding support for Bigquery will involve updating the secrets and can be done in the next round of changes.

To add additional adapter testing support in the future, you will just need to modify

• .github/workflows/ci_dbt_test_package.yml to pass through the env vars
• modify profiles.yml to meet the desire var format (example in [dbt-package-testing](https://github.com/dbt-labs/dbt-- package-testing/blob/main/integration_tests/profiles.yml))
• supported_adapters.env to list the adapters you want to test
• tox.ini to add the commands for the associated adapter tests

I've left your current testing as is to allow you to decide what to do with it.

Why Do We Need These Changes

As explained in #316, by setting up tox as a standard entry-point, the Core development team at dbt Labs can include this package's integration tests (along with other popular dbt packages) as part of our continuous development & delivery of dbt.

Checklist

• I have verified that these changes work locally
• I have updated the README.md (if applicable)
• I have added an integration test for my fix/feature (if applicable)

[emmyoop]

Can we hardcode this? Is it actually a secret?

[emmyoop]

To get this working I will need the following added as variables instead of secrets:

1. REDSHIFT_HOST
2. REDSHIFT_USER
3. REDSHIFT_PORT
4. REDSHIFT_DATABASE
5. SNOWFLAKE_USER
6. SNOWFLAKE_WAREHOUSE
7. SNOWFLAKE_ROLE
8. SNOWFLAKE_DATABASE
9. BIGQUERY_PROJECT

The following need to be added as secrets

1. DBT_ENV_SECRET_REDSHIFT_PASS
2. SNOWFLAKE_ACCOUNT
3. DBT_ENV_SECRET_SNOWFLAKE_PASS
4. BIGQUERY_KEYFILE_JSON