test: define constants for test images

[knqyf263]

Description

As GHCR rate limits may cause a migration to a different image repository, this PR defines the repository name in one place as a constant and it is referenced from both shell scripts and Go programs.

Related PRs

• test: use loaded image names #7617

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[DmitriyLewen]

LGTM