Testing standard libraries

[maleadt]

Julia apparently just supports testing (embedded) stdlibs via regular Pkg.add/Pkg.test, so we should be able to test those packages. We currently don't, because the stdlibs aren't all registered, so we don't consider them when determining the packages to test. But we can look them up using Pkg.Types.stdlibs().

Note that it would probably still be a bit of a special case because it wouldn't support the full generality of PackageSpec.