[wishlist] way to block certain workers from running certain tests

Some autopkgtest only fail on certain ci.d.n workers. Currently we only have the choose to either leave these flaky tests running until they are fixed, with the change that people need to look into the situation again to figure out why their upload caused a regression, or rejectlist them globally on the architecture involved. E.g. we had a bug where the test failed if run on a worker with a number of cores not equal to 2^n. On arm64 we currently have one worker with 48 cores, while the rest has 2.