This is relatively harmless, since we treat them *almost* identically,
except that we print a warning for overridden files to remind you that
something fishy is going on.
Add a test for the actual warning message to ensure it is printed. (I
don't like tests for specific warning messages, but it was necessary in
this case.)
Because the two programs use separate state databases, it helps if we
clean up some temp files between runs. Otherwise they might think you
created some targets "by hand" and refuse to rebuild them.
