test: expand unit and integration tests for broader coverage

[snabb]

Summary

• Add unit tests for ErrorWord, InterruptStatus, SelfTestResult, and interrupt_map_location
• Expand async I2C integration tests to cover self-test, enable_feature_engine, wait_for_interrupt, and error paths
• Expand blocking I2C integration tests with the same coverage

Test plan

• All existing tests continue to pass (cargo test)
• New tests cover previously uncovered paths (verify via cargo llvm-cov)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.13%. Comparing base (b86af30) to head (7c0b8f9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main       #6       +/-   ##
===========================================
+ Coverage   87.54%   99.13%   +11.59%     
===========================================
  Files           6        6               
  Lines        1846     1846               
===========================================
+ Hits         1616     1830      +214     
+ Misses        230       16      -214     

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