Minor update to streamline Dejagnu files with oban-scripts

This is only a few minor changes that remove some deviation between gnu-devtools and oban-scripts.

The changes in site-expand-list.exp are currently non-functional, as the two lists there are currently identical. This is likely the correct handling if the lists deviate in the future, though.

The change in site-exhaustive-qemu.exp adds a bit more test coverage and brings it inline with the FastModels exhaustive configuration.

As a separate point, I would suggest the team stop using "site-legacy-fastmodels" as soon as possible, in order to get better test coverage!

Change-Id: Id797bf6625d9be364fcfabe4fdfc7f4754e8ed76