...so clean up.  (Changing this might be tricky, esp. on Windows where the
python.exe wrapper adds the dev-install tree's dirs to PATH, so trying to run
tests against solver instead of dev-install tree libs will result in chaos.
This will become moot when we finally have solver == dev-install.)

Change-Id: Icbfc3928a7ff178fd86d78d15ffcf539557efbb4