Cover VOC 2007-test URL in download tests

[YeonwooSung]

Description

voc() in test/test_datasets_download.py had a TODO to also exercise the "2007-test" key. That archive is selected when year=="2007" and image_set=="test" (VOCtest_06-Nov-2007.tar).

This change:

• still collects trainval URLs for VOC 2007-2012
• also collects the 2007-test URL via VOCSegmentation(..., year="2007", image_set="test", download=True)
• adds an offline unit test that voc() includes DATASET_YEAR_DICT["2007-test"]["url"] and that the 2007/test constructor wiring selects that archive

The live voc() URL accessibility check stays disabled because the VOC server is unstable (see #2953).

Test plan

• pytest test/test_datasets_download.py -k "test_voc_includes_2007_test_url" (offline; no network)
• Do not re-enable the live voc() parametrization in test_url_is_accessible

[pytorch-bot]

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