Use a mock registry server in test suite

[afritzler]

What would you like to be added:

We are currently relying on a real OS image from ghcr.io to be pulled as part of the test suite. This creates an unnecessary external dependancy which might create frictions in test stability in the future.

Why is this needed:

In order to use a real image, we should rather start a localhost registry provider a dummy OCI manifest which can be fetched from the operator to extract the neccessary layers for the IPXEBootConfig and HTTPBootConfig.