Enable UBT test builds for libxml2

Overview

The Omnibus GitLab project must be able to consume components from UBT and build with them to validate the compiled outputs. To that end, a special target distribution exists in Omnibus GitLab where the software definitions may be changed to target output from UBT. The libxml2 build work is complete and it is ready to be included in this test definition.

Context

Each component built by UBT works in isolation. We need to be able to validate that a GitLab package built with these outputs can pass pipeline tests and offline tests. This is one of the final validation steps before all supported and shipped Linux packages may be changed over to UBT.

Deliverables

• Get package name for libxml2 and version in the general package registry for the UBT project.
• Amend the software definition to pull from and use the package for the libxml2 component. This should be conditional on the flag UBT_TEST_BUILD set to true, otherwise the software definition will build as it does today.
• Confirm the pipeline builds with UBT_TEST_BUILD set.