At some point the `a` mock package became an `AutotoolsPackage`, and that means it
depends on `gnuconfig` on macOS. This was causing one of our shell tests to fail on
macOS because it was testing for `{a.prefix.bin}:{b.prefix.bin}` in `PATH`, but
`gnuconfig` shows up between them.
- [x] simplify the test to check `spack load --sh a` and `spack load --sh b` separately