[CT-2898] [Feature] Enable merge_exclude_columns test for dbt-postgres

[dbeatty10]

Current Behavior

The source code here doesn't include TestMergeExcludeColumns like dbt-snowflake does here or dbt-redshift does here.

This is because merge wasn't a supported incremental strategy for dbt-postgres until 1.6 and adding relevant functional tests was overlooked during implementation in #6951.

Expected Behavior

e.g. I expected the end of the file to look like this:

class TestMergeExcludeColumns(BaseMergeExcludeColumns):
    pass

Steps To Reproduce

See above for context.

Relevant log output

No response

Environment

- OS:
- Python:
- dbt:

Which database adapter are you using with dbt?

No response

Additional Context

Due to a misunderstanding how this feature should behave, I originally thought the expected output should look like this, but I was mistaken! dbt-labs/dbt-redshift#564 brought dbt-redshift in line with the behavior of other adapters.

[github-actions]

This issue has been marked as Stale because it has been open for 180 days with no activity. If you would like the issue to remain open, please comment on the issue or else it will be closed in 7 days.