docs: add comprehensive testing documentation

[gadst12]

Summary

Adds docs/testing.md — documents VoteChain's full testing strategy including all test layers, test environment setup, commands for running each layer, coverage thresholds enforced by CI, and guidelines for writing new tests.

Changes

• docs/testing.md (new file, 411 lines)
  • Testing strategy overview table (unit, integration, property-based, TTL, load) with file locations and responsibilities
  • Test environment setup: Rust 1.75+, wasm32 target, no external services required; Docker option
  • Commands for: full suite, per-contract, specific test, with/without output, property-based, integration, load, and snapshot tests
  • Per-layer breakdown documenting exactly what each file covers (unit: 40+ governance tests, 20+ token tests; integration: 3 E2E scenarios; proptest: pass/reject formula invariants; TTL: persistent storage expiry; load: 1 000 proposals, 10 000 votes)
  • test_helpers.rs usage guide with example
  • Coverage: 80% line coverage minimum enforced in CI per package; cargo-llvm-cov commands for HTML and LCOV reports
  • CI jobs table: test, fmt-check, lint, coverage, security-audit, build-wasm, wasm-validity
  • Rules for writing new tests, naming convention, example unit test, example property-based test
  • Test snapshot update procedure

Acceptance criteria

• Testing docs describe test responsibilities per layer
• Commands for running tests are included
• Coverage and CI expectations are documented
• Any required test environment setup is described

Closes Vera3289#518