Add unit tests for isLooping() default, noLoop() and loop() states

[harshiltewari2004]

Resolves #8979

Changes

• Added a p5.prototype.isLooping suite to test/unit/core/structure.js covering the three states scoped in the issue: true by default, false after noLoop(), true again after loop().
• The suite creates its own p5 instance instead of using the file's shared one. Toggling noLoop()/loop() within the same frame on the shared instance double-pumps its draw loop — noLoop() doesn't cancel the pending requestAnimationFrame tick, and loop() immediately starts a new chain, so the stale tick resurrects alongside it. This made the existing timing-based "noLoop should stop" test fail (reproduced consistently; passes consistently without the new suite). A private instance isolates the toggling completely.
• The tests are synchronous, unlike the neighboring loop/noLoop tests: The tests are synchronous, unlike the neighboring loop/noLoop tests: those verify the draw loop's actual behavior, which needs real time to elapse so pending requestAnimationFrame ticks can fire (or fail to). These tests only assert the state flag, which noLoop()/loop() set synchronously before returning, so there's nothing to wait for.

PR Checklist

• npm run lint passes
• [Inline reference] is included / updated — N/A, test-only change
• [Unit tests] are included / updated

[lirenjie95]

LGTM once the placeholder in the PR description is cleaned up.

[harshiltewari2004]

Cleaned up-thanks for the catch!