Move mdocContext unit tests into mocha test suite.

[djscruggs]

Converts the standalone test/unit/mdocContext.test.js (added in #219) into a standard mocha suite at test/mocha/91-mdoc-context.js so it runs under the common testing system, npm test, and CI.

• Replaces the ad-hoc test()/assert() harness with describe/it and chai should assertions; test behaviors are unchanged (sign an mDL via mdocContext, verify against a trusted chain, reject an untrusted certificate).
• Hoists the repeated PEM-to-DER helper, device JWK, and signing boilerplate into shared helpers.
• Removes the test:unit script and the test/unit/ directory; no references remain.

There was no strong reason for the standalone runner beyond skipping Bedrock stack startup during development — --mocha-test covers that need without a parallel test system.

Closes #222.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.95%. Comparing base (1b033be) to head (7301bb9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #223   +/-   ##
=======================================
  Coverage   91.95%   91.95%           
=======================================
  Files          18       18           
  Lines        3678     3678           
=======================================
  Hits         3382     3382           
  Misses        296      296           

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