WebP thumbnail test fails on Meson builds

It seems to work with autotools builds. So it might be something else, maybe a lot of threads used for testing with Meson ? This happened after updating the Debian image that included libvpx. The test was likely not run before that.

Example: https://code.videolan.org/alexandre-janniaux/vlc/-/jobs/1481969

This might be related to #28359 (closed) which only fails for Meson builds as well.