Add feature policy tests for motion sensors

[2400030206]

Description

This PR adds comprehensive tests for the Accelerometer API, including:

• Basic sensor functionality for Accelerometer, GravitySensor, and LinearAccelerationSensor
• Feature Policy integration tests (enabled/disabled states)
• Cross-origin iframe accessibility tests
• Insecure context validation
• IDL harness compliance tests

Tests Added

• accelerometer/Accelerometer.https.html
• accelerometer/GravitySensor.https.html
• accelerometer/LinearAccelerationSensor.https.html
• accelerometer/Accelerometer_insecure_context.html
• accelerometer/Accelerometer-iframe-access.https.html
• Feature policy tests in accelerometer/*-by-feature-policy*.https.html
• IDL tests in accelerometer/idlharness.https.window.js

Manual Testing

• LinearAccelerationSensor shake threshold test requires manual device shaking
• All other tests are fully automated

Related Issues

Closes #[issue_number] - Add Accelerometer API test coverage

Reviewers

@riju @Honry @rakuco (from accelerometer/META.yml)

[mahesh1164]

Hi, I’d like to work on this issue for Hacktoberfest.
Please assign it to me if it’s still available. Thank you!

[gsnedders]

This is an issue and not a PR, and the overall format of this makes me feel like this is low-effort LLM attempt at something.