fix(transform): Add SkipMissingValues Flag to Test Transform

[jshlbrd]

Description

• Fixes an issue with test messages due to a missing flag

Motivation and Context

In v2.3.0 the Message package was refactored to use Flags for controlling transform behavior that was previously not configurable, but adding flags to the creation of test messages was missed. This doesn't impact production deployments, so it's not a breaking fix.

How Has This Been Tested?

Any config unit tests that fail in v2.3.0 due to missing values should now pass.

Types of changes

• 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.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.