efi: allow other architectures in check_host_security and friends

[alexclewontin]

Checking host security today makes assumes in many ways secboot will only be used on amd64. This PR attempts to lay the groundwork to allow other architectures to leverage much of the same logic.

Big themes:

• removing architecture-conditional compilation, to allow unit tests to be mocked and run on any host architecture
• abstracting the mocked test environment (the "fixture") from the actual code under test. This accounts for the bulk of the PR, as a ton of a couple test files now sit inside loops over fixtures. The whitespace insensitive diff is much smaller.
• adding switch/case statements to the business logic of checking host security, so that each architecture can implement its own ecosystem-aware checks (as even within amd64 there doesn't seem to be a platform agnostic way to handle this).

This is based on top of #562 and is part of an attempt to split up #560.