test: Add dvt_large_decimals tests for Db2

[nj1973]

Description of changes

Adding a missing test to Db2: dvt_large_decimals.

We can only test part of the table due to Db2 have a max decimal size of DECIMAL(31).

Issues to be closed

Closes #1653

Checklist

• I have followed the CONTRIBUTING Guide.
• I have commented my code, particularly in hard-to-understand areas
• I have updated any relevant documentation to reflect my changes, if applicable
• I have added unit and/or integration tests relevant to my change as needed
• I have already checked locally that all unit tests and linting are passing (use the tests/local_check.sh script)
• I have manually executed end-to-end testing (E2E) with the affected databases/engines

[nj1973]

/gcbrun