fix(test): isolate cache directory for downloader tests

[geertj]

Use os.MkdirTemp to create a unique temporary cache directory for each downloaderTest run instead of using a shared directory in $HOME. Creating a temporary directory under a fixed name in $HOME is unsafe and leads to test interference and resource conflicts when running tests in parallel.

TAG=agy
CONV=ac1f66fe-728b-45e3-88c1-8e5f724a1f6f

[gemini-code-assist]

Summary of Changes

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This pull request improves the reliability of the downloader test suite by eliminating potential resource conflicts and test interference. By moving away from a hardcoded directory in the user's home folder to dynamically generated temporary directories, the tests can now safely run in parallel without side effects.

Highlights

• Test Isolation: Replaced the shared global cache directory with unique temporary directories created via os.MkdirTemp for each test run.
• Resource Management: Updated the downloader test suite to manage its own cache directory lifecycle, ensuring proper cleanup during teardown.

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[gemini-code-assist]

Code Review

This pull request replaces the hardcoded global cacheDir with a dynamically generated temporary directory (dt.cacheDir) using os.MkdirTemp in the downloader tests, improving test isolation and cleanup. The reviewer suggests using AssertEq instead of ExpectEq when verifying the temporary directory creation to fail fast, and advises checking if dt.fakeStorage is nil in TearDown to prevent potential nil pointer panics if setup fails early.