[tests]: Move values of setup_header on stack

Calling read_unaligned requires an unsafe block. However it is safe to move the value of the required fields on the stack.

Before submitting your PR, please make sure you addressed the following requirements:

[x] All commits in this PR are signed (with git commit -s), and the commit message has max 60 characters for the summary and max 75 characters for each description line.
[x] All added/changed functionality has a corresponding unit/integration test.
[x] All added/changed public-facing functionality has entries in the "Upcoming Release" section of CHANGELOG.md (if no such section exists, please create one).
[x] Any newly added unsafe code is properly documented.

rust-vmm / linux-loader