[reconfigurator] Move planner tests to their corresponding submodules (#9042)

Like the Host OS
[tests](https://github.com/oxidecomputer/omicron/blob/main/nexus/reconfigurator/planning/src/mgs_updates/host_phase_1.rs#L539).
Tests that are specific to the SP, RoT and RoT bootloader should live in
their own submodules.

This PR is only moving code around. I have not made any changes to the
tests themselves

Author: karencfv