Create thumbs for a lot of MP4 videos with ffmpeg

I have a folder with 1000 MP4 videos and I would like to generate thumbnails for the videos.

Currently the videos are named like this:

test.mp4 test1.mp4 test2.mp4 test3.mp4

I would like the thumbnails images to be named like this:

test-thumb.jpg test1-thumb.jpg test2-thumb.jpg test3-thumb.jpg

Would you please help me with the right command to achieve that? I’m using CentOS 7.

for i in '' `seq 1 1000`;do
    ffmpeg -i test${i}.mp4 -vf  "thumbnail" -frames:v 1 test${i}-thumb.jpg
done