[kernel][phys] Add mexec variations of phys ZBI tests

This change introduces variations of the main phys ZBI tests in which
the ZBIs are booted via mexec.

Fixed: 60833
Change-Id: Ia6f58f1c11902aece795534b0af95d06994d3eb1
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/440189
Fuchsia-Auto-Submit: Joshua Seaton <joshuaseaton@google.com>
Commit-Queue: Joshua Seaton <joshuaseaton@google.com>
Reviewed-by: Roland McGrath <mcgrathr@google.com>
Reviewed-by: David Greenaway <dgreenaway@google.com>