tests: fixes uc24 nested tests - part 3

[sergiocazzolato]

This change includes more fixes for nested tests: . Missing models added . Changes preparing initramfs . New validation sets added . Changes in tests . Change in remote.pull to make it work in all the systems properly

These are the tests results before this change: https://paste.ubuntu.com/p/K4DrBxw45b/

This is the list of identified errors to fix in following prs: https://paste.ubuntu.com/p/5dnjmVmb7t/

[Meulengracht]

Thank you for posting the pre-PR test results

[sergiocazzolato]

These are the current results https://paste.ubuntu.com/p/nn5Rc3jYCd/

[zyga]

LGTM, thank you!