Add tests to validate behaviour of init-paging feature

[simongdavies]

This change will break https://github.com/hyperlight-dev/hyperlight-nanvix as Nanvix expects to boot from 0x0.

This was previously changed to 0x0 to support Nanvix here: f5ba6a5

I see that we're changing the base address to introduce the NULL guard page, but is this necessary for guest COW? If so, perhaps we can tie this change to init-paging too?

#[cfg(feature = "init-paging")]
pub(crate) const BASE_ADDRESS: usize = 0x1000;
#[cfg(not(feature = "init-paging"))]
pub(crate) const BASE_ADDRESS: usize = 0x0;

Originally posted by @danbugs in #1127 (comment)