tests(nano): increase coverage

[glevco]

Motivation

This PR improves test coverage of important nano or consensus related code, mostly by implementing new tests.

Acceptance Criteria

• Add # pragma: no cover comments on unreachable code branches.
• Add tests for allowed match syntax on OCBs.
• Add test for invalid Python syntax on OCBs.
• Add test for reorg with NC txs that have conflicts with other txs.
• Add missing tests for restricted signature mechanism on OCBs.

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[github-actions]

Bencher Report

Branch	tests/nano/increase-cov
Testbed	ubuntu-22.04

Click to view all benchmark results

Benchmark	Latency	Benchmark Result minutes (m) (Result Δ%)	Lower Boundary minutes (m) (Limit %)	Upper Boundary minutes (m) (Limit %)
sync-v2 (up to 20000 blocks)	📈 view plot 🚷 view threshold	2.00 m (+19.35%) Baseline: 1.67 m	1.51 m (75.41%)	2.01 m (99.46%)

🐰 View full continuous benchmarking report in Bencher

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.83%. Comparing base (b2775be) to head (074e74c).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1377      +/-   ##
==========================================
+ Coverage   85.68%   85.83%   +0.15%     
==========================================
  Files         429      429              
  Lines       32444    32423      -21     
  Branches     5066     5057       -9     
==========================================
+ Hits        27799    27830      +31     
+ Misses       3619     3583      -36     
+ Partials     1026     1010      -16     

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