Simplify regression test code.

[aaronweeden]

Description

This PR simplifies the regression test code by removing duplicated code in tandem with ubccr/xdmod#1815.

Motivation and Context

Code cleanup.

Tests performed

Types of changes

• Refactoring / documentation update (non-breaking change which does not change functionality)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project as found in the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.