FR: allow buildbot to read an 'exclude list' of tests from the Halide repo

[steven-johnson]

Currently, when we have to temporarily disable a test because of LLVM bugs, buildbot hardware issues, etc., we have to manually update the buildbot script. It would be preferable to have some sort of text file in the Halide repo itself that could be used to build up an 'exclude list', as updating the buildbot script is fiddly (and only a limited number of people have access to it), and having this in the Halide repo would allow individual PRs to tweak the excludes as needed.

[steven-johnson]

Related: CTest allows us to exclude tests via a series of regex, but our Makefile does not; we'd need similar support in Make for this feature to be genuinely useful.

[alexreinking]

TBQH we should rewrite the buildbot to read all of its steps from a JSON/YML/TOML (we can bikeshed this later) config file inside the repo.

[abadams]

Would certainly be nice to be able to update the way the buildbot behaves in the PR that necessitates the change.