remove videos from test for DatasetFolder

[pmeier]

The test mixed images and videos and this made it an outlier for two reasons:

1. It inherited from the image test case, but this meant it missed out on

   vision/test/datasets_utils.py

   Line 642 in c974742

   REQUIRED_PACKAGES = ("av",)

   coming from the video test case. This was not an issue until Add Python 3.11 Linux CPU Unittesting #7155 (comment).

2. Since the test needed to handle images and videos at the same time, we didn't bother to write an actual loader, but just returned the path back:

   vision/test/test_datasets.py

   Lines 1550 to 1551 in c974742

   	def dataset_args(self, tmpdir, config):
   	return tmpdir, lambda x: x

   Again, not an issue until Compatibility layer between stable datasets and prototype transforms #6663 (comment).

Both issues individually could be fixed, by adding the missing config parameter or by special casing it for the newly added tests. However, I don't think we gain much here by checking video and images.

Thus, this PR removes the video stuff from the test case and just focuses on the images. Note that in contrast to the test case for ImageFolder, we still test multiple extensions and make sure that we didn't pick up anything unrelated.

[NicolasHug]

So basically the test now checks that DatasetFolder focuses on the proper file extensions, and that the right loader is used? Sounds like this is enough for videos as well, even though we're not testing those directly anymore - LGTM!