refactor(device): move the host-switch session's tests to host_switch/tests.rs

host_switch.rs was 1041 lines, 400 of them an inline tests module. The module is host_switch/tests.rs now, beside the restore submodule that already lived there; `#[cfg(test)] mod tests;` stays where the inline module was.

Pure move: the module body is unchanged apart from one level of indentation; 21 tests before and after. host_switch.rs is 639 lines.