This commit adds `pkgs.test.cross.sanity`, which is meant to be a
carefully curated list of builds/packages that tend to break when
refactoring our cross-compilation infrastructure.
It should strike a balance between being small enough to fit in
a single eval (i.e. not so large that hydra-eval-jobs is needed)
so we can ask @ofborg to check it, yet should have good examples
of things that often break. So, no buckshot `mapTestOnCross`
calls here.
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