RFC: Add a .gitlab-ci.yml file for testing running tests with the current master version of dav1d

When adding new corner case testcases in the testdata repo, we want to make sure to merge them only once those tests pass on all supported architectures in CI, otherwise we will break the CI for the dav1d master branch. This allows a quick check for whether it's yet safe to merge new test cases (after merging fixes in dav1d master, one can rerun the CI jobs in this repo).

This kind of duplicates part of the dav1d CI setup (and needs to be kept kind updated in track with it), but I didn't include all configurations, primarily one per architecture. (This won't cover things like testing all possible x86 cpuflags where e.g. the ssse3 version could be broken while the avx2 one could be fine.)