Add new driver test for use of outdated .o files
This test ensures that it is not possible to trick GHC into reusing outdated .o
files if the .hi
file is up to date; something that doesn't currently appear to be being tested, since my branch for using content hashes for incremental builds is currently passing all of the other driver tests, but failing this one (as of hdgarrood/ghc@9aa337e4).