RFC: run pkg test on travis

This would catch at least some accidental breakage to pkg, such as #7493

This version slows Travis down a little more than it has to, since `test/pkg.jl` is executed separately from the rest of the tests which run in parallel. `runtests.jl` could instead be tweaked a little to include `pkg.jl` in its list under certain conditions, but cloning METADATA during the tests might not be acceptable for some distributions?