Later packages are supposed to shadow earlier ones in the stack,
unless the ordering is overriden with -package-id flags.
Unfortunately an earlier fix for something else had sorted the list of
packages so that it was in lexicographic order by installedPackageId,
and sadly our test (cabal/shadow) didn't pick this up because the
lexicographic ordering happened to work for the test.  I've now fixed
the test so it tries both orderings.