ci: Add CI for the V4L2 stateless decoders

This uses fluster in a virtual machine with the visl (VIrtual State Less) decoder driver to output frames without any decoder hardware.

It creates 4 tests (one for each supported decoder: vp8, vp9, h.264, h.265) that are part of the v4l2-stateless-decoders test suite.

A Gitlab CI job runs the whole suite.

Each test starts qemu with virtme and runs the test with the kernel configured with the kvm_kernel_path meson option. This kernel has the visl driver built-in and is built within the fedora docker image.

Some tests that seem to be failing are skipped, they can be reevaluated later if need be. Note that the virtual machine is running with only 8 smp cores even though the CI runner has 64. That is because some H.264 vectors tend to be unstable with more than 8 cores.

AV1 is currently not supported by visl but should be coming soon. Tests for it will be added then.