[Enhancement] Enable `adapter.get_columns_in_relation()` macro to return columns during unit testing

### Is this a new bug in dbt-core?

- [X] I believe this is a new bug in dbt-core
- [X] I have searched the existing issues, and I could not find an existing issue for this bug

### Current Behavior

The model I configured a unit test for, relies on the macro adapter.get_columns_in_relation() that I use for one of the models, referenced with the ref() macro. However, the macro adapter.get_columns_in_relation() returns an empty list for a relation during unit testing. This causes the SQL code to compile improperly and fail.

### Expected Behavior

The macro adapter.get_columns_in_relation() should return a list of columns for a referenced relation during unit testing.

### Steps To Reproduce

Create a SQL model that relies on the list of columns returned by the macro adapter.get_columns_in_relation(), then configure and run a unit test for this model

### Relevant log output

_No response_

### Environment

```markdown
- OS: Windows 10 Enterprise
- Python: 3.11.4
- dbt: 1.8.0
```

### Which database adapter are you using with dbt?

snowflake

### Additional Context

I hope it is possible to fix this, since dbt knows for sure the list of columns of all the referenced models. Apart from the input columns provided in the unit test config, dbt injects all other columns with NULL values into the resulting SQL for such a unit test. The input tables for a model, that I am trying to unit test, are really wide - 70-80 columns.