[coverage] Refactor download_fuzz_corpora.py for cross-platform support

Replace POSIX shell subprocess calls (mkdir, unzip, rm, tar) in
download_fuzz_corpora.py with Python standard library modules
(os.makedirs, zipfile, shutil). Add support for Windows .exe
fuzzer targets and update unit tests to run cross-platform.

Bug: 537414135, 538730493
Change-Id: I72bf804e8629286e0b06c5c6ab421ab980b3296e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8199897
Reviewed-by: Giovanni Ortuno Urquidi <ortuno@chromium.org>
Commit-Queue: David Hinojosa <davidhinojosa@google.com>
Reviewed-by: Jonathan Lee <jonathanjlee@google.com>
Cr-Commit-Position: refs/heads/main@{#1678242}
NOKEYCHECK=True
GitOrigin-RevId: 3e0a083998a4e7d4517d02021b9965bf44756091