iotests: Filter testfiles out in filter_img_info()

iotests: Filter testfiles out in filter_img_info()

We want to keep TEST_IMG for the full path of the main test image, but
filter_testfiles() must be called for other test images before replacing
other things like the image format because the test directory path could
contain the format as a substring.

Insert a filter_testfiles() call between both.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200424125448.63318-9-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>