[CT-2724] [Feature] Add negative test case for the split_part macro

[dbeatty10]

Is this your first time submitting a feature request?

• I have read the expectations for open source contributors
• I have searched the existing issues, and I could not find an existing issue for this feature
• I am requesting a straightforward extension of existing dbt functionality, rather than a Big Idea better suited to a discussion

Describe the feature

The issue described in dbt-labs/dbt-bigquery#615 is more general than dbt-bigquery, and the best way to support it is by adding a test cases for a negative index within dbt-core.

Describe alternatives you've considered

An option:

• each adapter maintainer is completely on their own to create test cases that include negative indices

This doesn't sound good for anyone, so let's go with Plan A!

Who will this benefit?

This will benefit both adapter maintainers and downstream users of the split_part macro.

Are you interested in contributing this feature?

🦞 is on it

Anything else?

No response