Test Suite: Add unit tests for Turtle class (part of #5607)

[AdityaM-IITH]

Adds initial unit test coverage for the Turtle class in js/turtle.js,
which previously had 0% test coverage.

Part of the ongoing test suite effort referenced in #5607.

Tests cover:

• blinking() — verifies correct boolean return based on _blinkFinished state
• doWait(secs) — verifies millisecond conversion including decimals and string input
• initTurtle(suppressOutput) — verifies correct initialization of 15 state variables
  including singer properties, timing values, and key signature defaults

Coverage: turtle.js statements increased from 0% to 34.15%

All 118 test suites passing locally.

@walterbender @omsuneri

[github-actions]

✅ All Jest tests passed! This PR is ready to merge.