Create shared TCB Module

[amd-aliem]

Context

Follow-up from PR #283 - (review thread from @DGonzalezVillal on snphost_config_commit.py).

TCB parsing, comparison, and to_u64 packing are already being reimplemented independently across 3 open PRs, and will keep recurring in every future TCB-touching test. Create a common module.

Preferred long-term direction - structured output from snpguest / sev crate

In my opinion we should improve snpguest to emit consumable structured output (e.g. snpguest display report --json, and similar for TCB) backed by the sev crate, and have the harness consume that. That, or have a rust->python binding for the sev crate.

• We currently re-define report fields the sev crate already models, so we don't exercise our own crate or the user-facing snpguest tool.
• Every new report version has to be re-implemented here in addition to the crate/tool - double maintenance, and a place for the two to drift.
• Parsing human-readable CLI output is brittle - raw-offset parsing (as pr#247 does) avoids that but moves the version/generation-tracking burden into the harness.

Proposal - short-term consolidation

Until structured snpguest output is available, collapse the three copies into a single shared module (e.g. sev_verify/cert_tests/common/tcb.py) rather than maintaining three.

[markg-github]

I agree and would add that (for json) the choice of json key names is important. Perhaps you could argue that sev/snpguest source code or snpguest display report --help output is the spec for this, but I would vote for using the names as they appear in the FW ABI spec.