test(downloader): scale down test object and chunk sizes to speed up tests

[geertj]

Optimized the test suite for the downloader package by scaling down the mock GCS object sizes and parallel/sparse download chunk sizes, reducing execution time from ~26s down to ~4.3s (a ~6x speedup).

Also fixed a race condition in Test_Download_CtxCancelled by reducing the timeout to 1ns to ensure immediate expiration and waiting on the async job completion channel (doneCh) before performing assertions on the downloaded file.

TAG=agy
CONV=8c7b5ab6-91ae-4a84-911e-1788917d5ab8

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request focuses on optimizing the downloader package's test suite to improve developer productivity by reducing execution time. It also includes a critical fix for a race condition in one of the asynchronous download tests to ensure test stability.

Highlights

• Test Optimization: Significantly reduced test execution time by scaling down mock GCS object sizes and download chunk sizes, resulting in a ~6x speedup.
• Race Condition Fix: Resolved a race condition in Test_Download_CtxCancelled by tightening the timeout and ensuring the async job completion channel is properly awaited before assertions.

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[gemini-code-assist]

Code Review

This pull request scales down the test object sizes, chunk sizes, and offsets across several downloader test files (job_test.go, parallel_downloads_job_test.go, and sparse_downloads_job_test.go) to optimize test execution. Additionally, it updates Test_Download_CtxCancelled to wait for the background async download to complete, preventing potential test flakiness. There are no review comments, so I have no feedback to provide.

The base branch was changed.

[raj-prince]

@geertj, tried triggering the CI checks from UI by changing the base branch, but got into bad state. Maybe you can try adding empty-commit once to re-trigger freshly.