video-tests: update download routine and add iMX8MM config
Updated the part of script that downloads the test videos from the provided URLs. The new version allows to set a custom name for output files. It is useful if filenames are the same but files are pulled from different sources. This renaming can be achieved by adding a custom name (separated by one or more spaces) after the URL. For example:
TEST_VIDEO_URLS="\
https://test-videos.co.uk/vids/bigbuckbunny/mp4/h264/1080/Big_Buck_Bunny_1080_10s_5MB.mp4 Big_Buck_Bunny_1080_10s_5MB_h264.mp4
https://test-videos.co.uk/vids/bigbuckbunny/mp4/h265/1080/Big_Buck_Bunny_1080_10s_5MB.mp4 Big_Buck_Bunny_1080_10s_5MB_h265.mp4
"
The commit also updates names of several variables to more accurately reflect their meaning.
Added config for iMX8MM boards.
Note: gst pipelines for imx8 were not tested due to lack of equipment