Add a simple test infrastructure to Yggdrasil

[vchuravy]

While chatting with Elliot we decided that we would be much more comfortable with rebuilding the world on occasion if we could make sue that things are at least loadable on x86.

Additionally this allows for consumers of JLLs to include a test/runtests.jl that should a run a very minimal set of smoke tests to gurantuee functionality.

[staticfloat]

[vchuravy]

Running on the public AZP runs into permission issues when hitting the cache.