Refactor Unit Tests

[rjzak]

The unit tests in src/main.rs are about the same size, or more, than the main Steward code. These should be refactored, minimized, or maybe moved to separate file(s) to make the code more manageable.

• Add test where a config-initialized Steward handle the requests.
• Only used the canned CSRs, which contained Enarx signatures.
• Finally test Steward in non-debug mode (related: chore: greater protection for release mode #72)